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Studies of atomic processes useful for atomic manipulation with STM
Materials Chemistry and Physics, 33 (1993) 1-14
1
Invited Review
Studies of atomic processes useful for atomic manipulation STM
with
Tien T. Tsong Institute of Physics, Academia Sinica, Nankzng, Taipei (Taiwan, ROC) (Received
June 6, 1992; accepted
June 26, 1992)
Abstract To create artificially designed structures of atomic size on the surface of a material, one can use the atomic scale mechanical maneuverability of the scanning tunneling microscope and the high electric field effects found in field ion microscopy. Although the simplest method one can use is perhaps the intrinsic force existing between an atom on the sample surface and an atom or a cluster on the tip surface, a force produced by an applied electric field can greatly facilitate the maneuverability. Effects of the applied field useful for atomic manipulation include field evaporation, field gradient-induced surface diffusion and tip cone formation, etc. A strong applied electric field can also induce a disordering of a surface or produce a new ordered surface atomic structure which is not found naturally. To create a thermally stable man-made structure, one may utilize an atomic replacement of the artificially placed adatoms with the substrate atoms. The replacement can occur for some refractory metals at or below room temperature.
1. Introduction It has been the dream of many scientists as well as engineers that one day it may be possible to manipulate on the 8, scale individual atoms on material surfaces. This is not only of great scientific interest [l], it may also one day enable us to create artificially designed material structures on the atomic scale, or to create new molecules which otherwise cannot be found in nature or be produced by other chemical methods [2-91. The prospect of achieving these goals looks brighter than ever with the recent demonstration by several investigators that atoms on a surface, either adsorbed ones or substrate atoms themselves, can be moved using the atomic scale mechanical maneuverability of the scanning tunneling microscope (STM), the intrinsic interactions of adsorbed atoms on the sample surface with the probing tip, and high electric field effects found in field ion microscopy (FIM). Studying the behavior of surface atoms and elementary atomic processes has been an endeavor of many investigators in FIM for many years. I will discuss some of the atomic processes
0254-0584/93/$6.00
studied with field ion microscopy (NM) which might be useful for atomic manipulation. Some of these FIM studies reveal in great detail the atomic steps and mechanisms and energetics of these atomic processes. I will also present a study of newly found atomic processes. They might be useful for atomic manipulation also.
2. Manipulation
by intrinsic
interactions
The most straight forward method for manipulating atoms with the STM is by the use of intrinsic interactions between an atom on the sample surface and an atom, a cluster, or the surface of the probing tip. This is the method used successfully by Eigler and coworkers [lo]. They find that an adsorbed atom can be moved from one location to another by either a horizontal sliding method or a vertical carrying method as sliding illustrated in Fig. 1. In the horizontal method, the probing tip of the STM is placed above an adsorbed atom. The tip is carefully lowered to increase the interaction between tip
0 1993 - Elsevier Sequoia. All rights reserved
2
-
Fig. 1. Methods for moving atoms by intrinsic interactions. (a) A horizontal sliding method. An ideal tip, which has a facet consisting of only one adsorption site, is first lowered to make light contact with an adatom. The tip is slid horizontal to the surface to a new location. The tip is lifted and the adatom is left at a new site of the sample surface. (b) A vertical carrying method. An ideal tip is lowered to make contact with an adatom. The tip is then lifted with the adatom adsorbed on the tip surface. The tip is moved to a desired position, and is lowered to make contact with the sample surface. Upon.Jifting, the atom is left on the surface at the new location. (c) The direction of the atom transfer can be better controlled if a voltage pulse is applied to either the tip or the sample. The polarity required depends on the atom in question as discussed in the section on field evaporation.
atoms and the adsorbed atom. After this, the tip is slowly moved in a direction parallel to the surface to a desired position. The adsorbed atom can be ‘dragged’ along. When the adatom is dragged to that position, the tip is lifted. This method seems to work well for physisorbed atoms as their interaction with the substrate is very weak. Eigler and Sweizer were able to use this method to rearrange condensed Xe atoms into a pattern of ‘IBM’ on a nickel surface. In the vertical carrying method, the probing tip is lowered to make direct ‘contact’, or to interact directly, with an adsorbed atom. When the tip is lifted, there is a chance the adatom is carried away by the tip. The tip is moved to a desired position and is then lowered to make ‘contact’ with the sample surface again. When the tip is lifted, there is a good chance the atom will be left on the sample surface at the new location. Thus the adatom is relocated. This method appears to work well for chemisorbed atoms such as metal adatoms. All these transfers of atoms are produced by intrinsic atomic interactions of the systems, often referred as ‘by the chemistry’ of the systems. The mechanisms of’the two methods are basically the same. Both of them use the interaction between the atom in question with the sample and tip surfaces as illustrated in Fig. 2. An atom can of
Fig. 2. When the tip to sample distance d is large, the atomtip and atom-sample interactions do not overlap significantly as shown in (a). When d is shortened, U,, which is the sum of U,, and U.,, exhibits a double well with a small activation barrier. The atom can transfer from the tip to the sample as well as from the sample to the tip as shown in (b). The transfer rate and the probabilities of finding the atom at the tip and sample surface are determined by the Boltzmann factors.
course interact simultaneously with both the tip surface and the sample surface. The potentials of these interactions are represented by U,, and U,,, respectively. When the atom is first deposited or condensed on the sample surface and the tip is placed very far away from the sample, the adsorbed atom interacts mainly with the substrate, or the atom sits inside the potential well of the adatomsample interaction U,,. As the tip is moved closer to the atom, the atom sees a potential equal to the sum of the two potentials, or U, = U,,+ U,,. At an appropriate distance, U, will exhibit a double well structure with a small activation barrier of heights Q0 and Q,’ from the tip side and from the sample side, respectively. At temperature T, the probabilities of finding the atom at the tip surface and the sample surface are exp(Q,,‘/k7’)/
[exp(Q&O + exp(QdWl
[exp(Q,/kT)
+ exp(Q,‘/kT)],
and
exp(Q&T)/
respectively. Obviously manipulations by the intrinsic interaction of a system depend on these probabilities but can be made easier by a clever choice of the tip and sample materials, or by careful consideration of the binding energies of the atom with the sample surface and with the tip surface. In principle, the manipulation can be controlled by an applied force field as will be discussed in the next section. In the vertical carrying method, the direction of the atom transfer (i.e. whether the atom is transferred from the sample surface to the tip or vice versa) can be controlled by the application of a small voltage pulse either to the tip or to the sample when the tip is brought to a near contact with the sample surface. This atomic transfer is pre-
3
sumably produced discussed below.
3. Manipulation
by field evaporation
as will be
by applied electric field
(al
n
3.1 Field evaporation and field desorption To transfer atoms from the sample to the tip or from the tip to the sample in the STM, one often uses a voltage bias or voltage pulses of a few tenths to a few volts having a range of widths of s to ns [2-91. These transfers are mainly produced by field evaporation, though greatly affected by intrinsic interactions. Field evaporation is a basic and fairly well studied phenomenon in field ion microscopy [ 1 l-131. Surface atoms, irrespective of whether they are lattice atoms or adsorbed atoms, can be field evaporated one at a time at very low temperature. Figure 3(a) gives an example of field evaporation of seven W adatoms, one by one, from a W (110) surface at 78 K by high voltage pulses. There are, however, a few differences between field evaporation in the STM configuration and that in the FIM configuration [6, 141. The fields required to field evaporate various materials in the STM configuration appear to be much lower than those required in the FIM configuration. Also many STM experiments can be more easily explained if field evaporation of negative ions is assumed. In FIM, as far as the author is aware of, no field evaporation of negative ions has been observed. I will try to explain these differences by extending the existing theoretical models of field evaporation in FIM to the conditions appropriate to the STM. In field evaporation, ions are produced by field ionization, or by an electronic transition between the atomic state and an ionic state, similar to ordinary field ionization of gases. The potential energy diagram for electrons in field ionization can be found in [13]. The critical fields required in field evaporation are, however, determined mainly by the potential energy change in the atomic and ionic states under the applied electric field, or by the transport of the atoms not the electrons. A potential energy diagram in ordinary field evaporation, i.e. evaporation of positive ions in a single electrode system of the FIM, is shown in Fig. 3(b). U, is the atomic potential, U,(O) is the ionic potential in zero field, and U,(F) is the ionic potential in the applied field F. At a very large distance from the surface, U;(O) is (C~li -n+) above the atomic state since Ci”li is needed to produce an ion of n + charge, but n+ is recovered when n electrons are returned to the surface at the Fermi
fi)
(ii)
(iii)
(iv)
(4
(vi)
(vii)
(viii)
(ix)
ut
(b) Xl-n@
I
L Z
Fig. 3. (a) Seven W atoms are vapor deposited on a small circular (110) facet of a W field ion emitter surface. Field ion images show field evaporation at 78 K of these seven W adatoms, one at a time, using high voltage pulses. Adatoms closer to the lattice steps are field evaporated earlier because of the higher field. (b) A potential energy diagram for field evaporation of metals in an isolated tip geometry of the FIM, based on the chargeexchange model. Details and parameters are explained in the text.
level. C#Jis the work function of the surface. A surface atom can be field evaporated if during its thermal motion it reaches beyond the intersection point of U, and U,(F) where U,(F) = Ui(0) - neFz. F is the applied field and z is the distance of the ion to the image plane. In low temperature field evaporation, the activation barrier Q,,(F) has to be very small, of the order 0.1 eV. Thus for all practical purposes, one may assume that field evaporation occurs at a field, F,,, when QJF,“) becomes zero. This also gives z, =r,, r, is ap-
4
proximately equal to the radius of the surface atom. If one approximates the ionic potential by UFP nze2/4r0- neFz where UareP is the repulsive part of the atomic potential, one easily arrives at
n$ 0
I
(1)
where A is the binding energy of the atom. The charge state n of the evaporated ions is the state which has the lowest F,,. This is also the evaporation field one should observe in FIM. This simple argument, based on the energetics of field evaporation, appears to be very much oversimplified. Yet it provides the only quantitatively reliable interpretation of the evaporation fields and charge states observed for most elements in field ion microscopy today. As an example, this calculation concludes that both Pt and Si should field evaporate as 2 + ions at a field of 4.5 V A- ’ and 3.6 V A-l, respectively (see Table 2.2 in [13a]). Experimental values are 4.8 V A-’ and 3.8 V A-l, respectively, and both elements indeed field evaporate as 2+ ions as can be seen in the time-of-flight atom-probe mass spectra shown in Fig. 4. In a two-electrode system such as the STM, the atom will interact with both the tip and the sample, so will the ion. The total atomic potential and ionic potential for a positive ion in zero field are shown in Fig. 5(a). Both of these potentials now depend on the separation between the two electrodes d. In fact if the distance d is small enough, the potential hump now existing in U, will be low enough so that direct transfer of atoms between the two electrodes by thermal activation will become possible as already discussed in Section 2. For an example, if the activation barrier is reduced to 0.772 eV, an atom transfer rate of KN 1 s-’ will occur between the two electrodes by thermal activation alone. In the applied field F, the ionic potential is modified by the addition of a term -neFz, and the potential energy curves now appear as shown in Fig. 5(b). For clarity and simplicity, we have omitted the polarization energy here. The evaporation field at an evaporation rate of K should now be given approximately by
-kTlnf
K
1
(2)
where v is the frequency factor. In Fig. 6, examples are given to show how the evaporation fields depend
FLItiHT
(4
TIME
I
,
I
I
( t.u.1
1
I
am-
tm28 *Em.
E -rl-
(b)
s
f*
Sll~con. 300 7000
K.
6 kV lxlOE(-9)torr
s1++
lOrlS
FLIGHTTIME (ns)
Fig. 4. (a) A time-of-flight spectrum showing spectral lines of the four major isotropes of Pt in low temperature (below 200 K) pulsed laser stimulated field evaporation. Only doubly charged Pt ions are detected. (b) A similar spectrum for Si. Again, only doubly-charged ions are detected. The line shape reflects the ion energy distribution. The true mass resolution is indicated by the inset placed above the spectrum. In a pulsed-laser stimulated high temperature field evaporation, Si atomic and cluster ions of l+ and 2+ charges can be detected.
on the electrode separation d. The evaporation fields required reduce to one half in the one electrode system when d is reduced to N 6 A. At this distance, a direct transfer of atoms by atomic interactions and thermal activation may already occur. Nevertheless, the field can still dictate the direction of the atom transfer. In field ion microscopy, no field evaporation in a negative applied field has either been observed or discussed before even though field. ionization of negative ions has been studied earlier [15]. The reason lies in the fact that evaporation fields of most materials exceed N 0.6 eV A- ’ when field electron emission current density will already be large enough to melt a slender tip of most materials used typically in field ion microscopy. With the
5
electrode geometry of the STM, field evaporation as negative ions may well become important. Melting of a tip becomes less important since either the scanning tip has a large facet with an adatom or a small atomic cluster on it, or the tip cone angle is very large. The tunneling current is also very much localized to a sub-nm region near the atom in question. To take an atom from a surface to free space requires an energy of A, the binding energy of the atom with the surface. To produce a negative ion of II - charge, we take IZ electrons from the surface at the Fermi level and attach it to the atom. This process requires an energy of (n+ -E,,“) where 4 is the work function of the surface and E affn is the (n -) charge electron affinity of the atom. In other words, to produce a negative ion of charge y1- from a surface in the absence of an electric field, an energy of (A +n4 - E,,“) is required. For comparison, the energy needed to produce a positive ion of charge 12+ requires energy of a (A+Di-n+). Th e ionic potential negative ion in a negative field is identical to that of a positive ion in a positive field. Theoretical models for ordinary field evaporation [14], the charge-exchange model [12] and the image-hump model [ll], can therefore be directly adopted to field evaporation of negative ions. All we have to do is replace (A + Cli -n$) with (A +n4 -E,f;). The evaporation fields of materials as negative ions in both the single and double electrode systems can be calculated using eqns. (1) and (2) after such a replacement. In Fig. 7, we show the fields required to field evaporate Cu, Ag, Au, Zn and Pb as 1 - ions at a rate of 1 s- ’ at 300 K in a negative field in the double-electrode system separated by 6 A based on this simple model. Negative ions of alkali metals and chemisorbed atoms of large electron affinity such as Cl, Br, or I, on metals of low work function can be produced by negative field evaporation. For the sake of comparison, the fields needed to field evaporate Na as Na+ and Na- at 1 s-’ at 300 K as functions of d in the double-electrode system are also shown. For systems of two electrodes of identical materials, the onset of field evaporation of either electrode will depend on which ion species has the lowest evaporation field (magnitude). In Table 1, the field strengths required to field evaporate various materials of common interest at a rate of 1 s- ’ at 300 K at an electrode separation of 6 8, as 1 +, 2+, 3 + and 1- ions are listed. From this table one can easily see that for alkali metals and a few soft materials such as Zn and Au, evaporation as negative ions requires the lowest
two
(b)
U
Fig. 5. (a) Ionic and atomic potentials in the STM configuration, but without an applied field. (b) Ionic and atomic potentials in the STM configuration when a positive field F is applied to the tip.
-
C-E
Model
----
I-H
Model
Fig. 6. The fields needed to field evaporate Au as Au+ and Au’+ and W as Wz+ and W3+ at a rate of 1 SC’ at 300 K as functions of the tip-sample distance in the STM. Solid lines are based on a charge-exchange model and dashed lines are based on an image hump model.
_~I
----
,
I
I
1
I
1
C.E Model I-H Model
DtSTANt”E
(A,
7. Similar curves for field evaporation of several metals as 1 - ions. For comparison, those for field evaporating as 1- and 1 + ions for Na are also shown.
Fig.
fields. For other soft metals such as Al, Cu, In, and Pb etc, the 1+ charge state is favored. For transition metals and Si and Ge, either 2+ or 3+ is favored. Although the method appears to be oversimpli~ed, we expect this calculation to be useful for predicting the charge state of the ion species and to be accurate to about f 15% in calculating the evaporation fields of metals, or having a similar accuracy as in field ion microscopy. The above calculations are used for illustrating the basic principles of field evaporation in the STM confi~ration, There, we assume the validity of the simple image potential in the double electrode system. Also we did not consider the possibility of field evaporation of multiple charged negative ions. In a recent more detailed calculation, Miskovsky and Tsong [16] used atomic energy curves computed from the embedded atom method 1171 and an effective binding potential method [18] and ionic curves computed from a multiple image approximation fl9]. They conclude that as the field is gradually raised, Au should field evaporate first as Au*- at a field close to 1 V A-“. In this calculation we realize that although Au2-
may not be a stable ion species in a field free region, in an applied field it can exist during the field evaporation transition within the region where this ion species has the lowest potential energy. As can be seen in Fig. 8(a), based on the embedded atom method, in an applied field of 1.1 V A-l, the ionic potential for Au*- is lower than that for either Au- or Au+. The activation energy of field evaporation as Au*- is the lowest. It is 0.772 eV at this field, or a field evaporation rate of - 1 The S -I can be expected at room temperature. effective binding potential method gives a very similar field of 0.9 V A-l as seen in Fig. S(b). The much lower field needed to field evaporate Au as Au*- appears to agree well with STM experiments. Aono et al. [9] argue that their data can be best interpreted in terms of field evaporation of Si- ions. Similar to gold, the critical field is much lower for Si2- than for other charge states, as shown by Miskovs~ et al. [16b]. 3.2 Field gradient induced sur~%cedijjksion In the absence of an external force, an adatom will perform random walk diffusion when the substrate temperature is raised. The atomic jumps are randomly directed. Random walk diffusion of single adsorbed atoms is a well known and well studied phenomenon [20]. What is less known is that in an applied force field, an adatom will still jump randomly but there will still be a net drift motion along the direction of the force, or the walk will become directional [21]. In a constant force the drifting velocity (6) can be shown to be given by (lj),-2
dexp(
- 2)
sinh(~)
(3)
where I is the jump distance, Ed is the activation energy in random walk diffusion, and f is the driving force. In field ion microscopy, because of the step structure of the emitter surface, the applied field is not uniform over a surface plane. The field near the edge of the plane is higher than that near the center of the plane. The field gradient, which can be measured from the position dependent evaporation field of vapor deposited adatoms on a surface plane, produces a location dependent binding potential by the addition of the polarization energy as shown in Fig. 9. Thus in an applied tip voltage, an adatom near the center of a net plane will always drift to the plane edge upon heating as can be seen in Fig. 10. The applied force in this problem is given by
7 TABLE
1. Evaporated
Element
fields (300 K,
1 s-‘) in STM configuration
A
(d=6
A)
Fl+ (V A-‘)
F2+ (V A-‘)
F3+ (V A-l)
(V A-‘)
Ion species LiB2+ Na-
IF’_1
Li Be Na Al Fe
1.65 3.33 1.13 3.34 4.29
2.5 3.9 2.3 4.1 4.4
1.56 1.13 1.91 1.43 1.27
5.39 9.32 5.14 5.99 7.90
75.64 18.21 47.29 18.83 16.16
122.45 153.89 71.64 28.45 30.65
0.62
0.42 3.59 0.23 0.81 2.69
20.65 2.92 9.72 2.09 1.73
36.07 40.40 16.17 3.22 3.32
0 5.13 0 4.07
$+
co Ni CU Zn MO
4.39 4.44 3.50 1.35 6.81
4.4 5.0 4.6 3.8 4.2
1.25 1.25 1.25 1.39 1.40
7.86 7.64 7.73 9.34 7.10
17.06 18.17 20.29 17.96 16.15
33.50 35.17 36.83 39.72 27.16
0.7 1.15 1.23 0 1.0
2.75 2.14 1.78 1.98 3.99
2.07 1.97 2.80 2.44 2.67
4.29 4.51 5.61 6.20 3.25
3.68 3.88 2.46 1.01 5.89
co2+ N?+ CU+ ZnMO’+
Rh Ag In cs Ta
5.75 2.96 2.60 0.83 8.09
4.8
(2) 4.2
1.35 1.45 1.66 2.73 1.47
7.46 7.57 5.79 3.89 7.89
18.08 21.49 18.87 25.1 16
31.06 34.83 28.03 35 22
1.2 1.3 0.3 0.47 0.6
3.11 j&O 0.37 b 5.28
2.65 3.09 1.96 2.82 3.35
4.02 5.24 3.20 4.43 2.59
5.14 2.21 2.53 0 7.6;
Ag* In+ csTa3 +
W Re Ir Pt AU
8.66 8.10 6.93 5.85 3.78
4.5 5.1 5.3 5.3 4.3
1.41 1.38 1.36 1.39 1.44
7.98 7.88 9.1 9.0 9.23
18 17 17 18.56 20.5
24 26 27 28 30
0.60 0.15 1.6 2.13 2.31
5.63 4.87 4.81 3.89 3.23
4.05 3.06 2.93 3.08 3.81
3.37 3.00 3.08 3.45 4.68
8.45 8.89 6.44 4.88 -1.71
W’+ Re3+ Re’+ I?+, ‘I?+ PtZ+ Au-
Pb C Si Ge
2.04 7.37 4.63 3.85
4.1 (4) (4.2) (4.2)
1.75 0.92 1.32 1.37
7.42 11.26 8.15 7.88
15.03 24.34 16.34 15.93
31.94 47.87 33.46 34.21
1.1 1.27 1.39 1.2
0.96 10.04 3.27 2.45
1.12 8.54 2.26 -1.78
3.28 13.22 4.48 4.34
1.36 4.79 3.17 2.67
Pb’ CS? + GeZC
He Ne Ar Kr Xe
-0 -0 -0 -0 -0
-4.5 - 4.5 -4.5 - 4.5 -4.5
1.1 1.58 1.88 2.00 2.17
24.6 21.6 15.8 14.0 12.1
-
,:p
where CL,,and cyare the static surface dipole and surface polarizability of the adatom, field gradient and F, is the field at the the net plane. It takes only a few steps that PO+ d,-
-
moment /3 is the center of to show
2kT pl sinh
where (p)r is the average drifting velocity and r is the length of the time period. In other words, by plotting the parameter y as a function of F,, a linear plot should be obtained. From the intercept and the slope of the plot, values of CL,,and (Ycan be derived. Such a plot has been obtained for W adatoms on the W (110) surface as shown in Fig. 11. Because of the large number of parameters involved, each of which has to be measured experimentally, the surface dipole moment and polarizability derived by this method are not accurate, but this is still a most direct method to provide such data of surface atoms.
13.9 8.4 4.1 3.0 2.0
-
_ _ _
2.90
Rh*+
HeNeArtiXe-
In the STM, the directional walk can be used to attract adsorbed atoms on the sample surface to the position directly below the probing tip as has been demonstrated by Whitman et al. [4] for Cs adatoms on the GaAs (110) and InSb (110) surfaces. At a low coverage of Cs atom adsorption on these surfaces, these atoms form sparsely distributed stable one-dimensional zigzag rows which can be imaged with a sample bias voltage of -2 to - 3 volts. When the biasing voltage is reversed to a positive voltage of - 1 volt for a short duration of time, 0.1 s to several tenths of a second, without moving the tip, the density of these atomic rows is found to increase dramatically at the position directly below the tip. This density increases with the duration of time of the reversed voltage. Figure 12 explains this observation. Because of the geometrical asymmetry of the tip-sample configuration, the field strength at the sample surface directly below the tip is higher than that away from the
8 EAM,
s=6
lo-
a-
\ \ I \
7-
‘,
9-
659 2.
43-
>
2-
z
‘-
1.
F=1.13
V/A,
.----
@=0.772
Atomic Ionic Ionic Ionic
__-_
’
Potential Potential Potential Potential
for for for
(a)=
/ I
‘1 \
/
‘.
\
Au’ AuAU’-
/
\ \ I I ’ \ \ ’i \ \
’\
eV
’
Centre
/
Edge
p=o E’
,I
1
-
1
/
’ (b)
-/s(p)
t -\ \
-41
cl*-
-5 -6
1 _71,,,,,,~,,,,,,,,,,,,,,,,,,,,,, 0 1
EBPM,
9-
aJ 7-
4
5
s=6
4-
z3
2
’
‘-
V/x,
Atomic Ionic Ionic Ionic
--
Q*-=0.772
Poientiol Potential Potential Potential
for for for
----
Au’ AuAU’-
I / I / J ’ /
’
\ \
\
’
’\
\
-1 -2
I
-.
\
‘,
’
’
-__/
(d)
1
/
‘\
/
I
‘\
/
‘.
‘.
-3
eV
\ kzf ‘\
2
F=0.903
-.--
I \
5-
t-
x,
\ \ /’ 1I
‘\ ’
6-
-\
‘-.._
,’
___I
.\
‘\
7
-4
d*-
-5 -6
‘l._____
z iA)
lo-
9 w
1 Cc)
2
(a)
2.
- + c@(P)
‘\\,
I
_71,,,,,,~,,,,,,,,,,,,,,,,,,,,,, 0 1
2
‘._/’ 4
// 5
i
6
z in,
(b)
Fig. 8. (a) The atomic curve of an Au atom and ionic curves with two Au (001) surfaces of AU+, Au- and AU=- Interacting . separated by 6 A in a double electrode system, based on an EAM. The applied field is 1.13 V A-‘. (b) Similar curves based on the EBPM at a field of 0.90 V A-‘. Note that in an applied field of - 1 V A-‘, the ionic curve of Au’- has the lowest energy among all these charge states even if the electron affinity of Au=- is taken to be -2 eV.
tip. Thus the directional walk of the surface atoms can occur. For simplicity we assume that the system is isotropic, or Al. and CYare constant and F(p) depends only on p, which is the distance to the central spot, or the spot directly below the tip. The potential energy of an adatom due to the applied field F(p) is then given by
U,(P)= -
EL*F(p) -
3 c@(p)
(6)
Fig. 9. When a voltage is applied to a field ion emitter, because of the step structure of the emitter facet, the strength of the applied field is not constant over the facet but varies with the location as illustrated in (a). The field is lowest at the central area of the facet and increases nearly linearly toward the step edge. (b) The surface potential of an adatom in the absence of an applied field is periodic except at the edge of the plane. Thus an adatom will perform a random walk on this surface, or the atomic jumps are randomly directed. (c) Under an applied voltage the potential energy of an adatom is reduced by the polarization energy term. Now the surface potential becomes inclined and the atomic jumps are preferentially directed toward the plane edge, or an adatom will drift from the central region of the surface directly toward the edge of the surface. (d) If the direction of the dipole moment of the adatom is parallel to the surface normal, then the effect of the dipole will be opposite to that of the induced dipole. Or in a negative field, the potential energy change will be very small because of the mutual cancellation of the dipole term and the induced dipole term.
where ,x is the dipole moment and (Y is the polarizability of the surface atom. Since CL is a vector, the effect of the applied field can depend on the polarity of the applied field. For those cases where p is parallel to the surface normal of the sample, a positive bias to the sample will lower the potential energy of the atom from both the dipole term and the induced dipole term. If the bias is reversed, the effect of the two terms will be opposite to one another and the potential energy change may reverse or may be too small
2.0
2.2
2.4
2.6
2.8
F,J’.‘IM
Fig. 11. A T-plot for W adatoms on the W (110) surface. From the slope, the polarizability of the adatoms is derived to be - 14 A’. From the intercept, the dipole moment of the adatoms is derived to be - 1.0 Debye.
Fig. 10. (a) Helium field ion images showing the directional walk of a W adatom on a W (112) surface at a F, of -4.5 V A-‘. The first picture is taken at 78 K. The rest are taken at -290 K. At 78 K, the W adatom cannot move. When the surface is heated to -290 K, the adatom starts to make one-dimensional random walk. From (ii) to (v), the adatom performs symmetric random walks since the field near the center of the facet is nearly constant. When the adatom jumps into the region where a field gradient exists, it starts to drift toward the step edge of the surface as shown in the fifth to the last picture. These time lapse images are separated by -2-5 s. Between two successive pictures is an atomic jump of the unit length, or the nearest neighbor distance of the W lattice 2.74 J%. (b) Similar images but for a W adatom on a W (110) surface. These images are taken with a He-Ar mixed gas. Without an applied field, diffusion on this surface is two dimensional. With an applied field, the walk is directional, or in the radial direction of the facet.
to produce a directional walk as seen in Fig. 9(d). From the fact that no directional walk is observed for a negative bias to the sample in their experiment, one can conclude that J.Lis parallel to the surface normal and the effect of the two terms nearly cancel each other in the field strength of the experiment.
Fig. 12. A diagram explaining why in an STM, when a positive bias is applied to the sample surface, adatoms on the surface will diffuse from the outer region toward the central spot directly below the tip. The field gradient at the sample surface comes from the geometrical asymmetry of the tip-sample surface configuration. The field at the central region is higher than away from this region.
The field gradient and temperature induced surface diffusion can also be used to produce a sharp tip as well as to form a cusp shaped sharp cone out of a dull tip. Atoms can be field evaporated from the tip apex and at the same time, a continuous flow of atoms from the tip shank to the tip apex can occur by the directional walk of surface atoms. If the temperature is high enough, the tip will become a liquid metal ion source where ions are continuously field evaporated from the tip apex and atoms are continuously supplied from the tip shank by a field gradient induced hydrodynamic flow of the liquid metal-like near-surface layer [22]. The phenomena are illustrated in Fig. 13.
3.3 Field induced surface atomic structures and structure disordering
(001) is found to have the (2X2) structure [23]. Below 2 V A-l, this superstructure is found to be disordered by the same heating of the surface. In the absence of an applied field, the thermally stable structure for this surface is the (1X 1) structure. In fact trying to induce a surface atomic rearrangement by pulser-laser heating below 2 V A-l results in the disordering of this surface. Examples are shown in Fig. 14. Similar observations have also been made in STM experiments. Whitman et al. [4] find that Cs atoms on the GaAs (110) surface form a closely packed 2 D structure phase upon applying a positive voltage bias to the sample surface. This structure is not found naturally. Without the positive biasing voltage, Cs atoms form one-dimensional chains on this surface. Aono et al. [24] on the other hand, find that a well ordered Si (111) (7 X 7) structure can be disordered by applying a high voltage bias to the sample surface during the tip scanning. It is therefore possible to create a new atomic structure of a surface or to disorder a surface by the application of an electric field. The structure created is not thermally stable once the applied field is removed, or the structure so created is only a metastable phase when the applied field is removed, but it
Surface atoms should interact with each other very differently from bulk atoms because surface atoms have very different bond configurations from bulk atoms. In other words, the crystal symmetry is broken at the surface. The (1 x 1) structure, which is the structure of a truncated surface of the solid, may not have the atomic arrangement of the lowest free energy at the surface. If the surface is carefully annealed to a temperature where atoms can rearrange, surface atomic reconstruction may occur to lower the free energy of the surface to the lowest free energy structure. Surface atomic reconstruction is a well known and well studied phenomenon in surface science. By the same token, in a strong applied electric field, the free energy of the surface can be changed by the polarization effect of the surface atoms. In other words, the atomic structure of a thermally annealed surface in zero field may not be the atomic arrangement of the lowest free energy in the presence of an applied high electric field. In an applied field at a temperature where atoms can rearrange, an atomic reconstruction may also occur. If the field is not high enough to produce a sufficient change in the polarization energy to stabilize the surface into a new ordered structure at that temperature, the surface may become disordered. In field ion microscopy, in a field above 3 V A-‘, the thermally stable surface of the Rh
Fig. 14. (a) A ~(2x2) Rh (001) structure is created by high temperature field evaporation at a field of -4.0 V A-‘. The temperature (-500 K) is produced by ns pulsed-laser heating. At this temperature, surface atoms have a sufficient mobility to rearrange themselves. When this surface is subjected further to the same heating at lower applied fields, the degree of order of the structure decreases with the applied fields as seen in (b), (c), and (d), where the fields are 3.2,2.0, and 0 VA-‘, respectively. All images are taken at -4.2 V A-‘.
(b)
Fig. 13. Because of a field gradient existing on a field emitter surface, when the tip is heated either by field electrons or by laser pulses, atoms will migrate from the tip shank to the tip apex continuously. Thus a cusp shape cone will be formed. If the field is high enough, atoms can be field evaporated from the tip apex. If the temperature is high enough, a liquid cone will be formed, and the supply of atoms will become a field gradient induced hydrodynamic flow of surface layers: This is a method which can be used to supply atoms to the sample indefinitely. If the tip-sample distance is too small, the liquid cone may touch the sample surface as illustrated in (b).
should be stable up to a threshold above which atomic rearrangements
4. Atomic replacement and thermally man-made atomic features
temperature can occur.
stable
When atoms are deposited on a surface either by vapor condensation or by field evaporation as described in Section 3.2, these adatoms may diffuse on the surface if the temperature of the surface is high enough. Diffusion of adatoms on perfect surfaces, even for surfaces of refractory metals can occur below room temperature [20]. Surface diffusion usually occurs by atomic hopping. For some surfaces such as Pt and Ir (001) and {llO}, self-diffusion can also occur by atomic replacement [25-291. For hetero-systems, if atomic replacement occurs, then the foreign adatoms are incorporated into the substrate or single atomic site alloying of the top surface layer occurs. Surface diffusion of these adatoms ceases [30]. Instead, self diffusion of the replaced substrate atoms will occur. A system showing these features very clearly is the Re/Ir (001). When a Re atom is deposited on a low temperature field evaporated Ir (001) surface, which has the (1 x 1) structure, its image appears completely normal, or it is circularly shaped as seen in Fig. 15(b). This adatom can be field evaporated at a field of about 4.5 + 0.5 VA-’ without producing an observable change of the substrate. To induce an atomic process, the surface is heated for a time period of - 10s. When the heating temperature is below 200 IS, no changes are observed. Around 220 K, we start to observe occasionally a sudden change of the Re adatom into a diatomic cluster, manifested by the change of the circular image spot to an oblong image spot, presumably by the formation of a Re-Ir dimer by activating a substrate atom onto the surface, as shown in Fig. 15(c). When the surface is heated again to 210-260 K, occasional changes of the orientation of the oblong shaped image spot can be seen (Fig. 15(c) to (f)). That a Re-Ir-vacancy complex is formed can be confirmed by the creation of a lattice vacancy in the surface layer directly beneath the dimer. Upon gradual low temperature field evaporation of the surface, which removes the cluster first and then atoms from the plane edges, a lattice vacancy is found to be left in the top surface layer at the position right beneath the Re-Ir dimer. This vacancy can already be seen before the receding edge of the layer reaches it, as shown in Fig. 16.
g
i
Fig. 1.5. (a) is an FIM image of a nearly circular top layer of an Ir (001) surface of diameter -60 A. [n (b), a Re atom is vapor deposited on this surface which appears circularly shaped. (c) When the sample is heated to -230 K, a Re-Ir dimer is formed which gives an oblong-shaped image. Upon heating to - 262 K, the orientation of the dimer changes by 90” occasionally as shown in (d) to (f). (g) to (i) illustrate these atomic processes.
Figure 16(g) to (i) illustrate the atomic processes involved [30]. The dimer-vacancy complex can be thermally dissociated if the heating temperature exceeds -280 K. Upon dissociation, the Re atom is found to fill the vacancy and the replaced Ir atom disappears from the top surface layer. The latter occurs because the diffusivity of Ir adatoms on the Ir (001) surface at such a temperature is very high and the plane boundary is not reflective to Ir adatoms [31]. The substitutional Re atom shows a slightly brighter image intensity than substrate atoms when the surface layer is reduced in size. It can be seen well before the plane edge reaches the Re atom as shown in Fig. 17(e) and (f). The activation barrier heights for these steps can be derived by measuring the temperature dependences of the average times of producing these atomic steps. The transition rate K is related to the average formation time 7 and the activation barrier height for the formation of the complex, & by K=
l/7= v exp( -E&T)
(7)
where v is the frequency factor, k the Boltzmann constant and T the temperature. By plotting the logarithm of the inverse of the average formation time against the inverse temperature, a straight
g
h
1
Fig. 16. (a) is an Ir (001) surface. (b) A Re adatom has been vapor deposited on this surface. For a very sharp tip, the FIM image of an adatom always appear to be excessively bright and fat. A Re-Ir dimer is formed by heating to 242 K as seen in (c). Upon gradual low temperature field evaporation of the dimer and the substrate layer, a lattice vacancy is found in the substrate directly beneath the dimer as seen in (e) and (f). (g) to (i) illustrate these atomic steps.
line should be obtained. Figure M(a) shows a set of data (solid line) we have obtained. From the slope and the intercept of this linear plot, the following data are derived: Ef= 0.65 t_ 0.05 eV, and v=5X10’2fl s-1 . For the reorientation and dissociation processes, Ef is replaced by E, and Edis. Two sets of data, shown also in Fig. 18(a), yield E,, = 0.57 f 0.07 eV and v= 8 X 101o*l for the reorientation process and Edis =0.95 kO.07 eV, and v=5X1()‘4*‘8-’ for the dissociation process. After considering some other observations, a potential energy diagram of the atomic replacement process of this system is obtained as shown in Fig. 18(b) 1301. With the above discussions, one should be able to produce a thermally stable top surface layer with an atomic pattern of substitutional foreign atoms of any design in it using a low temperature STM. Tip atoms can be deposited on the sample surface, one or a few at a time, by high temperature field evaporation of the tip. During the deposition, the tip will be further heated by the field emitted electrons. A temperature and field gradient induced surface diffusion will continue to supply atoms from the tip shank to the tip apex provided a proper experimental condition can be found.
9
h
Fig. 17. (a) to (c) are similar to Fig. 16(a) to (c). After a ReIr dimer-vacancy complex is formed, the surface is further heated to 280 K to thermally dissociate this complex. Both atoms disappear from the surface as seen in (d), but upon gradual low temperature field evaporation of the surface layer, the Re atom is found to have been incorporated into the substrate lattice at the original location. The substitutions Re atom is slightly brighter than the substrate fr atoms as seen in (e) and (f). (g) to (i) illustrate these atomic processes. The replaced Ir adatom diffuses to the lattice steps, falls of the step and is absorbed into the step.
Once a desired pattern of tip atoms are deposited on the sample surface, the sample can be pulse heated to -300 K to produce an atomic replacement as discussed earlier, thereby the deposited tip atoms and the neighbor substrate atoms are exchanged. Now the exchanged substrate atoms will have to be removed from the surface. This can be done by either the method described by Whitman et al. [4] or by using another tip with a strong tip-atom intera~tjon or by using field evaporation as in the vertical carrying method. The atomic-replacement surface diffusion can occur below 300 K for refractory metals. When the exchanged-substrate atoms are carefully removed, the embedded foreign atoms will not move until the bulk diffusion temperature is reached which is usually several hundred K. Thus the surface atomic pattern of substjtutjonal foreign atoms so designed and created shouid be thermally stable up to several hundred K. A possible procedure of this method is illustrated in Fig. 19.
13
Ind y
T-F
Ind
Diffu
Evap
(al
Atomic -7 -
-61 3.0
Replacement
Re/Ir(OOl) ’
(a)
*
I
4.0 1000/T
5.0
(K-‘) (b)
Ir lattice atom state
(b)
Dimer-vacancy s::
‘“ii’ Substitut. Ir adatom
Re + state
(c)
Fig. 18. (a) The average formation and dissociation times of the Re-Ir dimer-vacancy complex and the average reorientation time of the Re-Ir dimer sitting on the surface vacancy as functions of the temperature are plotted in the Arrhenius form. The activation energies and values of the pre-exponential factors of these rate equations are given in the text. (b) A potential energy diagram for the atomic-replacement reaction of Re/Ir{OOl}. The energies of the initial and final states, or the Re adatom plus Ir lattice atom state and the substitutional Re atom plus Ir adatom state, are not yet measured. Their relative positions are determined from the direction of the atomic replacement reactions.
Conclusions Atomic manipulation is still in its infancy. While the success of moving atoms around has captured the interest of many people, it is still a long way before one can expect any practical application. It appears though that the structures or molecules one could create with atomic and molecular manipulation will be limited only by our endeavors. In the meantime, it is important that we try to understand the physics of the basic atomic processes involved and useful for atomic manipulation.
References 1 For an example see R. P. Feynman, Also Science, 254 (1991). 2 R. S. Becker, J. A. Golovchenko and Nature, 325 (1987) 419.
Eng.
Sci.,
(1960)
B. S. Swartzentruber,
22.
Final
Surface
Fig. 19. Diagrams describing how a thermally stable atomic pattern of substitutional foreign atoms of one’s design can be created in the top surface layer of the sample by the high electric field effects and the atomic-replacement process described in the text. (a) Tip atoms are deposited on the sample surface by field evaporation. They are supplied continuously from the tip shank by a temperature and field gradient induced surface diffusion. (b) and (c) An atomic replacement reaction is then induced thereby the deposited tip atoms are incorporated into the substrate layer. This pattern of substitutional atoms should be thermally stable up to several hundred K.
3 D. M. Eigler and E. K. Sweizer, Nature, 344 (1990) 524. 4 L. J. Whitman, J. A. Stroscio, R. A. Dragoset, and R. J. Celotta, Science, 251 (1991) 1206. J. S. Foster, J. E. Frommer and P. C. Arnett, Nature, 331 (1988) 324. H. J. Mamin, P. H. Guethner and D. Rugar, Phys. Rev. Leir., 65 (1990). I. W. Lyo and P. Avouris, Science, 253 (1991) 173. For a recent review ofvarious methods of atomic manipulation, see J. A. Stroscio and D. M. Eigler, Science, 254 (1991) 1319 and references therein. F. Gray, R. S. Williams and A. Aono, to be 9 A. Kobayashi, published. 10 Ref. 3 and D. M. Eigler, C. P. Lutz and W. E. Rudge, Nature, 352 (1991) 600. 29 (1941) 533; Phys. Rev., 11 E. W. Mtiller, Nufurwissenshaften, 202 (1956) 618. 12 R. Gomer and L. W. Swanson, J. Chem. Phys., 38 (1963) 1613. Field Ion Microscopy, Cambridge 13 (a) T. T. Tsong, Arom-Probe University Press, Cambridge, 1990. (b) For field evaporation of adatoms see T. T. Tsong, J. Chem. Phys., 54 (1971) 4205. 14 T. T. Tsong, Phys. Rev., B44 (1991) 13703.
14 15 M. Anber and G. A. St. John, Science, 190 (1975) 781; V. A. Nazarenko and V. P. Pokhodenko, Int. J. Mass Spectrom. Zen Phys., 31 (1979) 381; R. Schmitz, L. Biiftering and Rollgen, J. Phys. COIL, C7 (1986) 53. 16 (a) N. M. Miskovsky and T. T. Tsong, Phys. Rev. B, 46 (1992) 2640; (b) N. M. Miskovsky, C. M. Wei and T. T. Tsong, Phys. Rev. Len., 69 (1992) 2427. 17 (a) M. S. Daw and M. I. Baskes, Phys. Rev. 829 (1984) 6443. (b) S. M. Foiles, M. I. Baskes and M. S. Daw, Phys. Rev., B33 (1986) 7983. 18 H. Gollisch, Surf: Sci. 166 (1986) 87; 175, idern, 249 (1986). 19 G. Binnig, N. Garcia, H. Rohrer, J. M. Soler and F. Flores, Phys. Rev., B30 (1984) 4816. 20 (a) G. Ehrlich and K. Stoltz,Ann. Rev. Phys. Chem., 31 (1980) 603. (b) T. T. Tsong, Prog. Surf: Sci., 10 (1980) 165; Rep. Prog. Phys., 51 (1988) 759.
21 T. T. Tsong and R. J. Walko, Physica Status Solidi, (a)12 (1972) 111; T. T. Tsong and G. L. Kellogg, Phys. Rev., B12 (1975) 1343; S. C. Wang and T. T. Tsong, Phys. Rev., B26 (1982) 6470. 22 T. T. Tsong, J. Chem. Phys., 54 (1971) 360 and references therein. 23 C. F. Ai and T. T. Tsong, Surf: Sci., 138 (1984) 339. 24 M. Aono, private communication. 25 D. W. Bassett, and P. R. Weber, Surf: Sci., 70 (1978) 520. 26 J. D. Wrigley and G. Ehrlich, Phys. Rev. Len., 44 (1980) 661. 27 G. L. Kellogg and P. J. Feibelman, Phys. Rev. Len,, 64 (1990) 3143. 28 P. J. Feibelman, Phys. Rev. Lett., 65 (1990) 729. 29 C. L. Chen and T. T. Tsong, Phys. Rev. Len., 64 (1990) 3147. 30 T. T. Tsong and C. L. Chen, Nature, 355 (1992) 328; C. L. Chen, T. T. Tsong, L. H. Zhang and Z. W. Yu, Phys. Rev. B, to be published. 31 T. T. Tsong and C. L. Chen, Phys. Rev., B43 (1991) 2007.
1
Invited Review
Studies of atomic processes useful for atomic manipulation STM
with
Tien T. Tsong Institute of Physics, Academia Sinica, Nankzng, Taipei (Taiwan, ROC) (Received
June 6, 1992; accepted
June 26, 1992)
Abstract To create artificially designed structures of atomic size on the surface of a material, one can use the atomic scale mechanical maneuverability of the scanning tunneling microscope and the high electric field effects found in field ion microscopy. Although the simplest method one can use is perhaps the intrinsic force existing between an atom on the sample surface and an atom or a cluster on the tip surface, a force produced by an applied electric field can greatly facilitate the maneuverability. Effects of the applied field useful for atomic manipulation include field evaporation, field gradient-induced surface diffusion and tip cone formation, etc. A strong applied electric field can also induce a disordering of a surface or produce a new ordered surface atomic structure which is not found naturally. To create a thermally stable man-made structure, one may utilize an atomic replacement of the artificially placed adatoms with the substrate atoms. The replacement can occur for some refractory metals at or below room temperature.
1. Introduction It has been the dream of many scientists as well as engineers that one day it may be possible to manipulate on the 8, scale individual atoms on material surfaces. This is not only of great scientific interest [l], it may also one day enable us to create artificially designed material structures on the atomic scale, or to create new molecules which otherwise cannot be found in nature or be produced by other chemical methods [2-91. The prospect of achieving these goals looks brighter than ever with the recent demonstration by several investigators that atoms on a surface, either adsorbed ones or substrate atoms themselves, can be moved using the atomic scale mechanical maneuverability of the scanning tunneling microscope (STM), the intrinsic interactions of adsorbed atoms on the sample surface with the probing tip, and high electric field effects found in field ion microscopy (FIM). Studying the behavior of surface atoms and elementary atomic processes has been an endeavor of many investigators in FIM for many years. I will discuss some of the atomic processes
0254-0584/93/$6.00
studied with field ion microscopy (NM) which might be useful for atomic manipulation. Some of these FIM studies reveal in great detail the atomic steps and mechanisms and energetics of these atomic processes. I will also present a study of newly found atomic processes. They might be useful for atomic manipulation also.
2. Manipulation
by intrinsic
interactions
The most straight forward method for manipulating atoms with the STM is by the use of intrinsic interactions between an atom on the sample surface and an atom, a cluster, or the surface of the probing tip. This is the method used successfully by Eigler and coworkers [lo]. They find that an adsorbed atom can be moved from one location to another by either a horizontal sliding method or a vertical carrying method as sliding illustrated in Fig. 1. In the horizontal method, the probing tip of the STM is placed above an adsorbed atom. The tip is carefully lowered to increase the interaction between tip
0 1993 - Elsevier Sequoia. All rights reserved
2
-
Fig. 1. Methods for moving atoms by intrinsic interactions. (a) A horizontal sliding method. An ideal tip, which has a facet consisting of only one adsorption site, is first lowered to make light contact with an adatom. The tip is slid horizontal to the surface to a new location. The tip is lifted and the adatom is left at a new site of the sample surface. (b) A vertical carrying method. An ideal tip is lowered to make contact with an adatom. The tip is then lifted with the adatom adsorbed on the tip surface. The tip is moved to a desired position, and is lowered to make contact with the sample surface. Upon.Jifting, the atom is left on the surface at the new location. (c) The direction of the atom transfer can be better controlled if a voltage pulse is applied to either the tip or the sample. The polarity required depends on the atom in question as discussed in the section on field evaporation.
atoms and the adsorbed atom. After this, the tip is slowly moved in a direction parallel to the surface to a desired position. The adsorbed atom can be ‘dragged’ along. When the adatom is dragged to that position, the tip is lifted. This method seems to work well for physisorbed atoms as their interaction with the substrate is very weak. Eigler and Sweizer were able to use this method to rearrange condensed Xe atoms into a pattern of ‘IBM’ on a nickel surface. In the vertical carrying method, the probing tip is lowered to make direct ‘contact’, or to interact directly, with an adsorbed atom. When the tip is lifted, there is a chance the adatom is carried away by the tip. The tip is moved to a desired position and is then lowered to make ‘contact’ with the sample surface again. When the tip is lifted, there is a good chance the atom will be left on the sample surface at the new location. Thus the adatom is relocated. This method appears to work well for chemisorbed atoms such as metal adatoms. All these transfers of atoms are produced by intrinsic atomic interactions of the systems, often referred as ‘by the chemistry’ of the systems. The mechanisms of’the two methods are basically the same. Both of them use the interaction between the atom in question with the sample and tip surfaces as illustrated in Fig. 2. An atom can of
Fig. 2. When the tip to sample distance d is large, the atomtip and atom-sample interactions do not overlap significantly as shown in (a). When d is shortened, U,, which is the sum of U,, and U.,, exhibits a double well with a small activation barrier. The atom can transfer from the tip to the sample as well as from the sample to the tip as shown in (b). The transfer rate and the probabilities of finding the atom at the tip and sample surface are determined by the Boltzmann factors.
course interact simultaneously with both the tip surface and the sample surface. The potentials of these interactions are represented by U,, and U,,, respectively. When the atom is first deposited or condensed on the sample surface and the tip is placed very far away from the sample, the adsorbed atom interacts mainly with the substrate, or the atom sits inside the potential well of the adatomsample interaction U,,. As the tip is moved closer to the atom, the atom sees a potential equal to the sum of the two potentials, or U, = U,,+ U,,. At an appropriate distance, U, will exhibit a double well structure with a small activation barrier of heights Q0 and Q,’ from the tip side and from the sample side, respectively. At temperature T, the probabilities of finding the atom at the tip surface and the sample surface are exp(Q,,‘/k7’)/
[exp(Q&O + exp(QdWl
[exp(Q,/kT)
+ exp(Q,‘/kT)],
and
exp(Q&T)/
respectively. Obviously manipulations by the intrinsic interaction of a system depend on these probabilities but can be made easier by a clever choice of the tip and sample materials, or by careful consideration of the binding energies of the atom with the sample surface and with the tip surface. In principle, the manipulation can be controlled by an applied force field as will be discussed in the next section. In the vertical carrying method, the direction of the atom transfer (i.e. whether the atom is transferred from the sample surface to the tip or vice versa) can be controlled by the application of a small voltage pulse either to the tip or to the sample when the tip is brought to a near contact with the sample surface. This atomic transfer is pre-
3
sumably produced discussed below.
3. Manipulation
by field evaporation
as will be
by applied electric field
(al
n
3.1 Field evaporation and field desorption To transfer atoms from the sample to the tip or from the tip to the sample in the STM, one often uses a voltage bias or voltage pulses of a few tenths to a few volts having a range of widths of s to ns [2-91. These transfers are mainly produced by field evaporation, though greatly affected by intrinsic interactions. Field evaporation is a basic and fairly well studied phenomenon in field ion microscopy [ 1 l-131. Surface atoms, irrespective of whether they are lattice atoms or adsorbed atoms, can be field evaporated one at a time at very low temperature. Figure 3(a) gives an example of field evaporation of seven W adatoms, one by one, from a W (110) surface at 78 K by high voltage pulses. There are, however, a few differences between field evaporation in the STM configuration and that in the FIM configuration [6, 141. The fields required to field evaporate various materials in the STM configuration appear to be much lower than those required in the FIM configuration. Also many STM experiments can be more easily explained if field evaporation of negative ions is assumed. In FIM, as far as the author is aware of, no field evaporation of negative ions has been observed. I will try to explain these differences by extending the existing theoretical models of field evaporation in FIM to the conditions appropriate to the STM. In field evaporation, ions are produced by field ionization, or by an electronic transition between the atomic state and an ionic state, similar to ordinary field ionization of gases. The potential energy diagram for electrons in field ionization can be found in [13]. The critical fields required in field evaporation are, however, determined mainly by the potential energy change in the atomic and ionic states under the applied electric field, or by the transport of the atoms not the electrons. A potential energy diagram in ordinary field evaporation, i.e. evaporation of positive ions in a single electrode system of the FIM, is shown in Fig. 3(b). U, is the atomic potential, U,(O) is the ionic potential in zero field, and U,(F) is the ionic potential in the applied field F. At a very large distance from the surface, U;(O) is (C~li -n+) above the atomic state since Ci”li is needed to produce an ion of n + charge, but n+ is recovered when n electrons are returned to the surface at the Fermi
fi)
(ii)
(iii)
(iv)
(4
(vi)
(vii)
(viii)
(ix)
ut
(b) Xl-n@
I
L Z
Fig. 3. (a) Seven W atoms are vapor deposited on a small circular (110) facet of a W field ion emitter surface. Field ion images show field evaporation at 78 K of these seven W adatoms, one at a time, using high voltage pulses. Adatoms closer to the lattice steps are field evaporated earlier because of the higher field. (b) A potential energy diagram for field evaporation of metals in an isolated tip geometry of the FIM, based on the chargeexchange model. Details and parameters are explained in the text.
level. C#Jis the work function of the surface. A surface atom can be field evaporated if during its thermal motion it reaches beyond the intersection point of U, and U,(F) where U,(F) = Ui(0) - neFz. F is the applied field and z is the distance of the ion to the image plane. In low temperature field evaporation, the activation barrier Q,,(F) has to be very small, of the order 0.1 eV. Thus for all practical purposes, one may assume that field evaporation occurs at a field, F,,, when QJF,“) becomes zero. This also gives z, =r,, r, is ap-
4
proximately equal to the radius of the surface atom. If one approximates the ionic potential by UFP nze2/4r0- neFz where UareP is the repulsive part of the atomic potential, one easily arrives at
n$ 0
I
(1)
where A is the binding energy of the atom. The charge state n of the evaporated ions is the state which has the lowest F,,. This is also the evaporation field one should observe in FIM. This simple argument, based on the energetics of field evaporation, appears to be very much oversimplified. Yet it provides the only quantitatively reliable interpretation of the evaporation fields and charge states observed for most elements in field ion microscopy today. As an example, this calculation concludes that both Pt and Si should field evaporate as 2 + ions at a field of 4.5 V A- ’ and 3.6 V A-l, respectively (see Table 2.2 in [13a]). Experimental values are 4.8 V A-’ and 3.8 V A-l, respectively, and both elements indeed field evaporate as 2+ ions as can be seen in the time-of-flight atom-probe mass spectra shown in Fig. 4. In a two-electrode system such as the STM, the atom will interact with both the tip and the sample, so will the ion. The total atomic potential and ionic potential for a positive ion in zero field are shown in Fig. 5(a). Both of these potentials now depend on the separation between the two electrodes d. In fact if the distance d is small enough, the potential hump now existing in U, will be low enough so that direct transfer of atoms between the two electrodes by thermal activation will become possible as already discussed in Section 2. For an example, if the activation barrier is reduced to 0.772 eV, an atom transfer rate of KN 1 s-’ will occur between the two electrodes by thermal activation alone. In the applied field F, the ionic potential is modified by the addition of a term -neFz, and the potential energy curves now appear as shown in Fig. 5(b). For clarity and simplicity, we have omitted the polarization energy here. The evaporation field at an evaporation rate of K should now be given approximately by
-kTlnf
K
1
(2)
where v is the frequency factor. In Fig. 6, examples are given to show how the evaporation fields depend
FLItiHT
(4
TIME
I
,
I
I
( t.u.1
1
I
am-
tm28 *Em.
E -rl-
(b)
s
f*
Sll~con. 300 7000
K.
6 kV lxlOE(-9)torr
s1++
lOrlS
FLIGHTTIME (ns)
Fig. 4. (a) A time-of-flight spectrum showing spectral lines of the four major isotropes of Pt in low temperature (below 200 K) pulsed laser stimulated field evaporation. Only doubly charged Pt ions are detected. (b) A similar spectrum for Si. Again, only doubly-charged ions are detected. The line shape reflects the ion energy distribution. The true mass resolution is indicated by the inset placed above the spectrum. In a pulsed-laser stimulated high temperature field evaporation, Si atomic and cluster ions of l+ and 2+ charges can be detected.
on the electrode separation d. The evaporation fields required reduce to one half in the one electrode system when d is reduced to N 6 A. At this distance, a direct transfer of atoms by atomic interactions and thermal activation may already occur. Nevertheless, the field can still dictate the direction of the atom transfer. In field ion microscopy, no field evaporation in a negative applied field has either been observed or discussed before even though field. ionization of negative ions has been studied earlier [15]. The reason lies in the fact that evaporation fields of most materials exceed N 0.6 eV A- ’ when field electron emission current density will already be large enough to melt a slender tip of most materials used typically in field ion microscopy. With the
5
electrode geometry of the STM, field evaporation as negative ions may well become important. Melting of a tip becomes less important since either the scanning tip has a large facet with an adatom or a small atomic cluster on it, or the tip cone angle is very large. The tunneling current is also very much localized to a sub-nm region near the atom in question. To take an atom from a surface to free space requires an energy of A, the binding energy of the atom with the surface. To produce a negative ion of II - charge, we take IZ electrons from the surface at the Fermi level and attach it to the atom. This process requires an energy of (n+ -E,,“) where 4 is the work function of the surface and E affn is the (n -) charge electron affinity of the atom. In other words, to produce a negative ion of charge y1- from a surface in the absence of an electric field, an energy of (A +n4 - E,,“) is required. For comparison, the energy needed to produce a positive ion of charge 12+ requires energy of a (A+Di-n+). Th e ionic potential negative ion in a negative field is identical to that of a positive ion in a positive field. Theoretical models for ordinary field evaporation [14], the charge-exchange model [12] and the image-hump model [ll], can therefore be directly adopted to field evaporation of negative ions. All we have to do is replace (A + Cli -n$) with (A +n4 -E,f;). The evaporation fields of materials as negative ions in both the single and double electrode systems can be calculated using eqns. (1) and (2) after such a replacement. In Fig. 7, we show the fields required to field evaporate Cu, Ag, Au, Zn and Pb as 1 - ions at a rate of 1 s- ’ at 300 K in a negative field in the double-electrode system separated by 6 A based on this simple model. Negative ions of alkali metals and chemisorbed atoms of large electron affinity such as Cl, Br, or I, on metals of low work function can be produced by negative field evaporation. For the sake of comparison, the fields needed to field evaporate Na as Na+ and Na- at 1 s-’ at 300 K as functions of d in the double-electrode system are also shown. For systems of two electrodes of identical materials, the onset of field evaporation of either electrode will depend on which ion species has the lowest evaporation field (magnitude). In Table 1, the field strengths required to field evaporate various materials of common interest at a rate of 1 s- ’ at 300 K at an electrode separation of 6 8, as 1 +, 2+, 3 + and 1- ions are listed. From this table one can easily see that for alkali metals and a few soft materials such as Zn and Au, evaporation as negative ions requires the lowest
two
(b)
U
Fig. 5. (a) Ionic and atomic potentials in the STM configuration, but without an applied field. (b) Ionic and atomic potentials in the STM configuration when a positive field F is applied to the tip.
-
C-E
Model
----
I-H
Model
Fig. 6. The fields needed to field evaporate Au as Au+ and Au’+ and W as Wz+ and W3+ at a rate of 1 SC’ at 300 K as functions of the tip-sample distance in the STM. Solid lines are based on a charge-exchange model and dashed lines are based on an image hump model.
_~I
----
,
I
I
1
I
1
C.E Model I-H Model
DtSTANt”E
(A,
7. Similar curves for field evaporation of several metals as 1 - ions. For comparison, those for field evaporating as 1- and 1 + ions for Na are also shown.
Fig.
fields. For other soft metals such as Al, Cu, In, and Pb etc, the 1+ charge state is favored. For transition metals and Si and Ge, either 2+ or 3+ is favored. Although the method appears to be oversimpli~ed, we expect this calculation to be useful for predicting the charge state of the ion species and to be accurate to about f 15% in calculating the evaporation fields of metals, or having a similar accuracy as in field ion microscopy. The above calculations are used for illustrating the basic principles of field evaporation in the STM confi~ration, There, we assume the validity of the simple image potential in the double electrode system. Also we did not consider the possibility of field evaporation of multiple charged negative ions. In a recent more detailed calculation, Miskovsky and Tsong [16] used atomic energy curves computed from the embedded atom method 1171 and an effective binding potential method [18] and ionic curves computed from a multiple image approximation fl9]. They conclude that as the field is gradually raised, Au should field evaporate first as Au*- at a field close to 1 V A-“. In this calculation we realize that although Au2-
may not be a stable ion species in a field free region, in an applied field it can exist during the field evaporation transition within the region where this ion species has the lowest potential energy. As can be seen in Fig. 8(a), based on the embedded atom method, in an applied field of 1.1 V A-l, the ionic potential for Au*- is lower than that for either Au- or Au+. The activation energy of field evaporation as Au*- is the lowest. It is 0.772 eV at this field, or a field evaporation rate of - 1 The S -I can be expected at room temperature. effective binding potential method gives a very similar field of 0.9 V A-l as seen in Fig. S(b). The much lower field needed to field evaporate Au as Au*- appears to agree well with STM experiments. Aono et al. [9] argue that their data can be best interpreted in terms of field evaporation of Si- ions. Similar to gold, the critical field is much lower for Si2- than for other charge states, as shown by Miskovs~ et al. [16b]. 3.2 Field gradient induced sur~%cedijjksion In the absence of an external force, an adatom will perform random walk diffusion when the substrate temperature is raised. The atomic jumps are randomly directed. Random walk diffusion of single adsorbed atoms is a well known and well studied phenomenon [20]. What is less known is that in an applied force field, an adatom will still jump randomly but there will still be a net drift motion along the direction of the force, or the walk will become directional [21]. In a constant force the drifting velocity (6) can be shown to be given by (lj),-2
dexp(
- 2)
sinh(~)
(3)
where I is the jump distance, Ed is the activation energy in random walk diffusion, and f is the driving force. In field ion microscopy, because of the step structure of the emitter surface, the applied field is not uniform over a surface plane. The field near the edge of the plane is higher than that near the center of the plane. The field gradient, which can be measured from the position dependent evaporation field of vapor deposited adatoms on a surface plane, produces a location dependent binding potential by the addition of the polarization energy as shown in Fig. 9. Thus in an applied tip voltage, an adatom near the center of a net plane will always drift to the plane edge upon heating as can be seen in Fig. 10. The applied force in this problem is given by
7 TABLE
1. Evaporated
Element
fields (300 K,
1 s-‘) in STM configuration
A
(d=6
A)
Fl+ (V A-‘)
F2+ (V A-‘)
F3+ (V A-l)
(V A-‘)
Ion species LiB2+ Na-
IF’_1
Li Be Na Al Fe
1.65 3.33 1.13 3.34 4.29
2.5 3.9 2.3 4.1 4.4
1.56 1.13 1.91 1.43 1.27
5.39 9.32 5.14 5.99 7.90
75.64 18.21 47.29 18.83 16.16
122.45 153.89 71.64 28.45 30.65
0.62
0.42 3.59 0.23 0.81 2.69
20.65 2.92 9.72 2.09 1.73
36.07 40.40 16.17 3.22 3.32
0 5.13 0 4.07
$+
co Ni CU Zn MO
4.39 4.44 3.50 1.35 6.81
4.4 5.0 4.6 3.8 4.2
1.25 1.25 1.25 1.39 1.40
7.86 7.64 7.73 9.34 7.10
17.06 18.17 20.29 17.96 16.15
33.50 35.17 36.83 39.72 27.16
0.7 1.15 1.23 0 1.0
2.75 2.14 1.78 1.98 3.99
2.07 1.97 2.80 2.44 2.67
4.29 4.51 5.61 6.20 3.25
3.68 3.88 2.46 1.01 5.89
co2+ N?+ CU+ ZnMO’+
Rh Ag In cs Ta
5.75 2.96 2.60 0.83 8.09
4.8
(2) 4.2
1.35 1.45 1.66 2.73 1.47
7.46 7.57 5.79 3.89 7.89
18.08 21.49 18.87 25.1 16
31.06 34.83 28.03 35 22
1.2 1.3 0.3 0.47 0.6
3.11 j&O 0.37 b 5.28
2.65 3.09 1.96 2.82 3.35
4.02 5.24 3.20 4.43 2.59
5.14 2.21 2.53 0 7.6;
Ag* In+ csTa3 +
W Re Ir Pt AU
8.66 8.10 6.93 5.85 3.78
4.5 5.1 5.3 5.3 4.3
1.41 1.38 1.36 1.39 1.44
7.98 7.88 9.1 9.0 9.23
18 17 17 18.56 20.5
24 26 27 28 30
0.60 0.15 1.6 2.13 2.31
5.63 4.87 4.81 3.89 3.23
4.05 3.06 2.93 3.08 3.81
3.37 3.00 3.08 3.45 4.68
8.45 8.89 6.44 4.88 -1.71
W’+ Re3+ Re’+ I?+, ‘I?+ PtZ+ Au-
Pb C Si Ge
2.04 7.37 4.63 3.85
4.1 (4) (4.2) (4.2)
1.75 0.92 1.32 1.37
7.42 11.26 8.15 7.88
15.03 24.34 16.34 15.93
31.94 47.87 33.46 34.21
1.1 1.27 1.39 1.2
0.96 10.04 3.27 2.45
1.12 8.54 2.26 -1.78
3.28 13.22 4.48 4.34
1.36 4.79 3.17 2.67
Pb’ CS? + GeZC
He Ne Ar Kr Xe
-0 -0 -0 -0 -0
-4.5 - 4.5 -4.5 - 4.5 -4.5
1.1 1.58 1.88 2.00 2.17
24.6 21.6 15.8 14.0 12.1
-
,:p
where CL,,and cyare the static surface dipole and surface polarizability of the adatom, field gradient and F, is the field at the the net plane. It takes only a few steps that PO+ d,-
-
moment /3 is the center of to show
2kT pl sinh
where (p)r is the average drifting velocity and r is the length of the time period. In other words, by plotting the parameter y as a function of F,, a linear plot should be obtained. From the intercept and the slope of the plot, values of CL,,and (Ycan be derived. Such a plot has been obtained for W adatoms on the W (110) surface as shown in Fig. 11. Because of the large number of parameters involved, each of which has to be measured experimentally, the surface dipole moment and polarizability derived by this method are not accurate, but this is still a most direct method to provide such data of surface atoms.
13.9 8.4 4.1 3.0 2.0
-
_ _ _
2.90
Rh*+
HeNeArtiXe-
In the STM, the directional walk can be used to attract adsorbed atoms on the sample surface to the position directly below the probing tip as has been demonstrated by Whitman et al. [4] for Cs adatoms on the GaAs (110) and InSb (110) surfaces. At a low coverage of Cs atom adsorption on these surfaces, these atoms form sparsely distributed stable one-dimensional zigzag rows which can be imaged with a sample bias voltage of -2 to - 3 volts. When the biasing voltage is reversed to a positive voltage of - 1 volt for a short duration of time, 0.1 s to several tenths of a second, without moving the tip, the density of these atomic rows is found to increase dramatically at the position directly below the tip. This density increases with the duration of time of the reversed voltage. Figure 12 explains this observation. Because of the geometrical asymmetry of the tip-sample configuration, the field strength at the sample surface directly below the tip is higher than that away from the
8 EAM,
s=6
lo-
a-
\ \ I \
7-
‘,
9-
659 2.
43-
>
2-
z
‘-
1.
F=1.13
V/A,
.----
@=0.772
Atomic Ionic Ionic Ionic
__-_
’
Potential Potential Potential Potential
for for for
(a)=
/ I
‘1 \
/
‘.
\
Au’ AuAU’-
/
\ \ I I ’ \ \ ’i \ \
’\
eV
’
Centre
/
Edge
p=o E’
,I
1
-
1
/
’ (b)
-/s(p)
t -\ \
-41
cl*-
-5 -6
1 _71,,,,,,~,,,,,,,,,,,,,,,,,,,,,, 0 1
EBPM,
9-
aJ 7-
4
5
s=6
4-
z3
2
’
‘-
V/x,
Atomic Ionic Ionic Ionic
--
Q*-=0.772
Poientiol Potential Potential Potential
for for for
----
Au’ AuAU’-
I / I / J ’ /
’
\ \
\
’
’\
\
-1 -2
I
-.
\
‘,
’
’
-__/
(d)
1
/
‘\
/
I
‘\
/
‘.
‘.
-3
eV
\ kzf ‘\
2
F=0.903
-.--
I \
5-
t-
x,
\ \ /’ 1I
‘\ ’
6-
-\
‘-.._
,’
___I
.\
‘\
7
-4
d*-
-5 -6
‘l._____
z iA)
lo-
9 w
1 Cc)
2
(a)
2.
- + c@(P)
‘\\,
I
_71,,,,,,~,,,,,,,,,,,,,,,,,,,,,, 0 1
2
‘._/’ 4
// 5
i
6
z in,
(b)
Fig. 8. (a) The atomic curve of an Au atom and ionic curves with two Au (001) surfaces of AU+, Au- and AU=- Interacting . separated by 6 A in a double electrode system, based on an EAM. The applied field is 1.13 V A-‘. (b) Similar curves based on the EBPM at a field of 0.90 V A-‘. Note that in an applied field of - 1 V A-‘, the ionic curve of Au’- has the lowest energy among all these charge states even if the electron affinity of Au=- is taken to be -2 eV.
tip. Thus the directional walk of the surface atoms can occur. For simplicity we assume that the system is isotropic, or Al. and CYare constant and F(p) depends only on p, which is the distance to the central spot, or the spot directly below the tip. The potential energy of an adatom due to the applied field F(p) is then given by
U,(P)= -
EL*F(p) -
3 c@(p)
(6)
Fig. 9. When a voltage is applied to a field ion emitter, because of the step structure of the emitter facet, the strength of the applied field is not constant over the facet but varies with the location as illustrated in (a). The field is lowest at the central area of the facet and increases nearly linearly toward the step edge. (b) The surface potential of an adatom in the absence of an applied field is periodic except at the edge of the plane. Thus an adatom will perform a random walk on this surface, or the atomic jumps are randomly directed. (c) Under an applied voltage the potential energy of an adatom is reduced by the polarization energy term. Now the surface potential becomes inclined and the atomic jumps are preferentially directed toward the plane edge, or an adatom will drift from the central region of the surface directly toward the edge of the surface. (d) If the direction of the dipole moment of the adatom is parallel to the surface normal, then the effect of the dipole will be opposite to that of the induced dipole. Or in a negative field, the potential energy change will be very small because of the mutual cancellation of the dipole term and the induced dipole term.
where ,x is the dipole moment and (Y is the polarizability of the surface atom. Since CL is a vector, the effect of the applied field can depend on the polarity of the applied field. For those cases where p is parallel to the surface normal of the sample, a positive bias to the sample will lower the potential energy of the atom from both the dipole term and the induced dipole term. If the bias is reversed, the effect of the two terms will be opposite to one another and the potential energy change may reverse or may be too small
2.0
2.2
2.4
2.6
2.8
F,J’.‘IM
Fig. 11. A T-plot for W adatoms on the W (110) surface. From the slope, the polarizability of the adatoms is derived to be - 14 A’. From the intercept, the dipole moment of the adatoms is derived to be - 1.0 Debye.
Fig. 10. (a) Helium field ion images showing the directional walk of a W adatom on a W (112) surface at a F, of -4.5 V A-‘. The first picture is taken at 78 K. The rest are taken at -290 K. At 78 K, the W adatom cannot move. When the surface is heated to -290 K, the adatom starts to make one-dimensional random walk. From (ii) to (v), the adatom performs symmetric random walks since the field near the center of the facet is nearly constant. When the adatom jumps into the region where a field gradient exists, it starts to drift toward the step edge of the surface as shown in the fifth to the last picture. These time lapse images are separated by -2-5 s. Between two successive pictures is an atomic jump of the unit length, or the nearest neighbor distance of the W lattice 2.74 J%. (b) Similar images but for a W adatom on a W (110) surface. These images are taken with a He-Ar mixed gas. Without an applied field, diffusion on this surface is two dimensional. With an applied field, the walk is directional, or in the radial direction of the facet.
to produce a directional walk as seen in Fig. 9(d). From the fact that no directional walk is observed for a negative bias to the sample in their experiment, one can conclude that J.Lis parallel to the surface normal and the effect of the two terms nearly cancel each other in the field strength of the experiment.
Fig. 12. A diagram explaining why in an STM, when a positive bias is applied to the sample surface, adatoms on the surface will diffuse from the outer region toward the central spot directly below the tip. The field gradient at the sample surface comes from the geometrical asymmetry of the tip-sample surface configuration. The field at the central region is higher than away from this region.
The field gradient and temperature induced surface diffusion can also be used to produce a sharp tip as well as to form a cusp shaped sharp cone out of a dull tip. Atoms can be field evaporated from the tip apex and at the same time, a continuous flow of atoms from the tip shank to the tip apex can occur by the directional walk of surface atoms. If the temperature is high enough, the tip will become a liquid metal ion source where ions are continuously field evaporated from the tip apex and atoms are continuously supplied from the tip shank by a field gradient induced hydrodynamic flow of the liquid metal-like near-surface layer [22]. The phenomena are illustrated in Fig. 13.
3.3 Field induced surface atomic structures and structure disordering
(001) is found to have the (2X2) structure [23]. Below 2 V A-l, this superstructure is found to be disordered by the same heating of the surface. In the absence of an applied field, the thermally stable structure for this surface is the (1X 1) structure. In fact trying to induce a surface atomic rearrangement by pulser-laser heating below 2 V A-l results in the disordering of this surface. Examples are shown in Fig. 14. Similar observations have also been made in STM experiments. Whitman et al. [4] find that Cs atoms on the GaAs (110) surface form a closely packed 2 D structure phase upon applying a positive voltage bias to the sample surface. This structure is not found naturally. Without the positive biasing voltage, Cs atoms form one-dimensional chains on this surface. Aono et al. [24] on the other hand, find that a well ordered Si (111) (7 X 7) structure can be disordered by applying a high voltage bias to the sample surface during the tip scanning. It is therefore possible to create a new atomic structure of a surface or to disorder a surface by the application of an electric field. The structure created is not thermally stable once the applied field is removed, or the structure so created is only a metastable phase when the applied field is removed, but it
Surface atoms should interact with each other very differently from bulk atoms because surface atoms have very different bond configurations from bulk atoms. In other words, the crystal symmetry is broken at the surface. The (1 x 1) structure, which is the structure of a truncated surface of the solid, may not have the atomic arrangement of the lowest free energy at the surface. If the surface is carefully annealed to a temperature where atoms can rearrange, surface atomic reconstruction may occur to lower the free energy of the surface to the lowest free energy structure. Surface atomic reconstruction is a well known and well studied phenomenon in surface science. By the same token, in a strong applied electric field, the free energy of the surface can be changed by the polarization effect of the surface atoms. In other words, the atomic structure of a thermally annealed surface in zero field may not be the atomic arrangement of the lowest free energy in the presence of an applied high electric field. In an applied field at a temperature where atoms can rearrange, an atomic reconstruction may also occur. If the field is not high enough to produce a sufficient change in the polarization energy to stabilize the surface into a new ordered structure at that temperature, the surface may become disordered. In field ion microscopy, in a field above 3 V A-‘, the thermally stable surface of the Rh
Fig. 14. (a) A ~(2x2) Rh (001) structure is created by high temperature field evaporation at a field of -4.0 V A-‘. The temperature (-500 K) is produced by ns pulsed-laser heating. At this temperature, surface atoms have a sufficient mobility to rearrange themselves. When this surface is subjected further to the same heating at lower applied fields, the degree of order of the structure decreases with the applied fields as seen in (b), (c), and (d), where the fields are 3.2,2.0, and 0 VA-‘, respectively. All images are taken at -4.2 V A-‘.
(b)
Fig. 13. Because of a field gradient existing on a field emitter surface, when the tip is heated either by field electrons or by laser pulses, atoms will migrate from the tip shank to the tip apex continuously. Thus a cusp shape cone will be formed. If the field is high enough, atoms can be field evaporated from the tip apex. If the temperature is high enough, a liquid cone will be formed, and the supply of atoms will become a field gradient induced hydrodynamic flow of surface layers: This is a method which can be used to supply atoms to the sample indefinitely. If the tip-sample distance is too small, the liquid cone may touch the sample surface as illustrated in (b).
should be stable up to a threshold above which atomic rearrangements
4. Atomic replacement and thermally man-made atomic features
temperature can occur.
stable
When atoms are deposited on a surface either by vapor condensation or by field evaporation as described in Section 3.2, these adatoms may diffuse on the surface if the temperature of the surface is high enough. Diffusion of adatoms on perfect surfaces, even for surfaces of refractory metals can occur below room temperature [20]. Surface diffusion usually occurs by atomic hopping. For some surfaces such as Pt and Ir (001) and {llO}, self-diffusion can also occur by atomic replacement [25-291. For hetero-systems, if atomic replacement occurs, then the foreign adatoms are incorporated into the substrate or single atomic site alloying of the top surface layer occurs. Surface diffusion of these adatoms ceases [30]. Instead, self diffusion of the replaced substrate atoms will occur. A system showing these features very clearly is the Re/Ir (001). When a Re atom is deposited on a low temperature field evaporated Ir (001) surface, which has the (1 x 1) structure, its image appears completely normal, or it is circularly shaped as seen in Fig. 15(b). This adatom can be field evaporated at a field of about 4.5 + 0.5 VA-’ without producing an observable change of the substrate. To induce an atomic process, the surface is heated for a time period of - 10s. When the heating temperature is below 200 IS, no changes are observed. Around 220 K, we start to observe occasionally a sudden change of the Re adatom into a diatomic cluster, manifested by the change of the circular image spot to an oblong image spot, presumably by the formation of a Re-Ir dimer by activating a substrate atom onto the surface, as shown in Fig. 15(c). When the surface is heated again to 210-260 K, occasional changes of the orientation of the oblong shaped image spot can be seen (Fig. 15(c) to (f)). That a Re-Ir-vacancy complex is formed can be confirmed by the creation of a lattice vacancy in the surface layer directly beneath the dimer. Upon gradual low temperature field evaporation of the surface, which removes the cluster first and then atoms from the plane edges, a lattice vacancy is found to be left in the top surface layer at the position right beneath the Re-Ir dimer. This vacancy can already be seen before the receding edge of the layer reaches it, as shown in Fig. 16.
g
i
Fig. 1.5. (a) is an FIM image of a nearly circular top layer of an Ir (001) surface of diameter -60 A. [n (b), a Re atom is vapor deposited on this surface which appears circularly shaped. (c) When the sample is heated to -230 K, a Re-Ir dimer is formed which gives an oblong-shaped image. Upon heating to - 262 K, the orientation of the dimer changes by 90” occasionally as shown in (d) to (f). (g) to (i) illustrate these atomic processes.
Figure 16(g) to (i) illustrate the atomic processes involved [30]. The dimer-vacancy complex can be thermally dissociated if the heating temperature exceeds -280 K. Upon dissociation, the Re atom is found to fill the vacancy and the replaced Ir atom disappears from the top surface layer. The latter occurs because the diffusivity of Ir adatoms on the Ir (001) surface at such a temperature is very high and the plane boundary is not reflective to Ir adatoms [31]. The substitutional Re atom shows a slightly brighter image intensity than substrate atoms when the surface layer is reduced in size. It can be seen well before the plane edge reaches the Re atom as shown in Fig. 17(e) and (f). The activation barrier heights for these steps can be derived by measuring the temperature dependences of the average times of producing these atomic steps. The transition rate K is related to the average formation time 7 and the activation barrier height for the formation of the complex, & by K=
l/7= v exp( -E&T)
(7)
where v is the frequency factor, k the Boltzmann constant and T the temperature. By plotting the logarithm of the inverse of the average formation time against the inverse temperature, a straight
g
h
1
Fig. 16. (a) is an Ir (001) surface. (b) A Re adatom has been vapor deposited on this surface. For a very sharp tip, the FIM image of an adatom always appear to be excessively bright and fat. A Re-Ir dimer is formed by heating to 242 K as seen in (c). Upon gradual low temperature field evaporation of the dimer and the substrate layer, a lattice vacancy is found in the substrate directly beneath the dimer as seen in (e) and (f). (g) to (i) illustrate these atomic steps.
line should be obtained. Figure M(a) shows a set of data (solid line) we have obtained. From the slope and the intercept of this linear plot, the following data are derived: Ef= 0.65 t_ 0.05 eV, and v=5X10’2fl s-1 . For the reorientation and dissociation processes, Ef is replaced by E, and Edis. Two sets of data, shown also in Fig. 18(a), yield E,, = 0.57 f 0.07 eV and v= 8 X 101o*l for the reorientation process and Edis =0.95 kO.07 eV, and v=5X1()‘4*‘8-’ for the dissociation process. After considering some other observations, a potential energy diagram of the atomic replacement process of this system is obtained as shown in Fig. 18(b) 1301. With the above discussions, one should be able to produce a thermally stable top surface layer with an atomic pattern of substitutional foreign atoms of any design in it using a low temperature STM. Tip atoms can be deposited on the sample surface, one or a few at a time, by high temperature field evaporation of the tip. During the deposition, the tip will be further heated by the field emitted electrons. A temperature and field gradient induced surface diffusion will continue to supply atoms from the tip shank to the tip apex provided a proper experimental condition can be found.
9
h
Fig. 17. (a) to (c) are similar to Fig. 16(a) to (c). After a ReIr dimer-vacancy complex is formed, the surface is further heated to 280 K to thermally dissociate this complex. Both atoms disappear from the surface as seen in (d), but upon gradual low temperature field evaporation of the surface layer, the Re atom is found to have been incorporated into the substrate lattice at the original location. The substitutions Re atom is slightly brighter than the substrate fr atoms as seen in (e) and (f). (g) to (i) illustrate these atomic processes. The replaced Ir adatom diffuses to the lattice steps, falls of the step and is absorbed into the step.
Once a desired pattern of tip atoms are deposited on the sample surface, the sample can be pulse heated to -300 K to produce an atomic replacement as discussed earlier, thereby the deposited tip atoms and the neighbor substrate atoms are exchanged. Now the exchanged substrate atoms will have to be removed from the surface. This can be done by either the method described by Whitman et al. [4] or by using another tip with a strong tip-atom intera~tjon or by using field evaporation as in the vertical carrying method. The atomic-replacement surface diffusion can occur below 300 K for refractory metals. When the exchanged-substrate atoms are carefully removed, the embedded foreign atoms will not move until the bulk diffusion temperature is reached which is usually several hundred K. Thus the surface atomic pattern of substjtutjonal foreign atoms so designed and created shouid be thermally stable up to several hundred K. A possible procedure of this method is illustrated in Fig. 19.
13
Ind y
T-F
Ind
Diffu
Evap
(al
Atomic -7 -
-61 3.0
Replacement
Re/Ir(OOl) ’
(a)
*
I
4.0 1000/T
5.0
(K-‘) (b)
Ir lattice atom state
(b)
Dimer-vacancy s::
‘“ii’ Substitut. Ir adatom
Re + state
(c)
Fig. 18. (a) The average formation and dissociation times of the Re-Ir dimer-vacancy complex and the average reorientation time of the Re-Ir dimer sitting on the surface vacancy as functions of the temperature are plotted in the Arrhenius form. The activation energies and values of the pre-exponential factors of these rate equations are given in the text. (b) A potential energy diagram for the atomic-replacement reaction of Re/Ir{OOl}. The energies of the initial and final states, or the Re adatom plus Ir lattice atom state and the substitutional Re atom plus Ir adatom state, are not yet measured. Their relative positions are determined from the direction of the atomic replacement reactions.
Conclusions Atomic manipulation is still in its infancy. While the success of moving atoms around has captured the interest of many people, it is still a long way before one can expect any practical application. It appears though that the structures or molecules one could create with atomic and molecular manipulation will be limited only by our endeavors. In the meantime, it is important that we try to understand the physics of the basic atomic processes involved and useful for atomic manipulation.
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Eng.
Sci.,
(1960)
B. S. Swartzentruber,
22.
Final
Surface
Fig. 19. Diagrams describing how a thermally stable atomic pattern of substitutional foreign atoms of one’s design can be created in the top surface layer of the sample by the high electric field effects and the atomic-replacement process described in the text. (a) Tip atoms are deposited on the sample surface by field evaporation. They are supplied continuously from the tip shank by a temperature and field gradient induced surface diffusion. (b) and (c) An atomic replacement reaction is then induced thereby the deposited tip atoms are incorporated into the substrate layer. This pattern of substitutional atoms should be thermally stable up to several hundred K.
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