Scanning tunneling topography and spectroscopy of silver on graphite

Thin Solid Films 256 (1995) 268-272

Scanning tunneling topography T. David,

and spectroscopy

J.P. Dufour,

of silver on graphite

G. Chabrier

Laboratoire de Physique du Solide, associti au CNRS, Fuc. Sciences Mirande, BP 138, 21004 Dijon cedex, France Received

24 May

1994; accepted

3 August

1994

Abstract This paper presents some experimental results obtained with silver films and particles evaporated on graphite. We have recorded I,- V, characteristics in air and in vacuum on identical samples. The most important difference between the two families of characteristics is the existence of differential negative resistance in air. This evidences the role of the surrounding samples in the formation of the tunnel current. Keywords:

Nanostructures;

Scanning

tunnelling

microscopy;

Silver

here our first results obtained with silver films and silver particles of nanometric scale deposited onto graphite.

1. Introduction Scanning tunnelling microscopy (STM) is now a usual surface analysis technique for obtaining data in real space and electronic structure, ultimately with atomic resolution. However, theoretical modelling of the tip and sample barrier and of the derivation of the tunnel current have not been rigorously solved. The first relevant calculation in this regard was published by Feuchtwang [ 1] about ten years before the first application of STM by Binnig et al. [2]. Bardeen’s transfer Hamiltonian formalism [3] has led to Tersoff and Hamann’s model [4] which is suitable when tip and sample are weakly coupled. Several groups have formulated more exact theories to consider the strong coupling regime. Sacks and Noguera [ 51, Pendry et al. [6], and Lucas et al. [7] derive an exact formula by solving the Lippman-Schwinger equation of the scattering problem for the true tip-sample barrier from the sum of all orders of perturbation of the wave function of the separated tip and sample. In the weak coupling limit their results are the same as in the transfer Hamiltonian formalism. The tunnelling current Z, is then proportional to the sample local density of states (LDOS) and the tip-sample bias polarization V,. Large variations in the tunnelling current are expected if the tip is probing atoms of different species and in the Tersoff and Hamann approximation Louis et al. [ 81 have shown that the derivative dZ,/d I’, is directly related to the sample local density of states. We present 0040-6090/95/$9.50 0 1995 ~ Elsevier XSDlOO40-6090(94)06314-l

Science S.A. All rights

reserved

2. Experimental

process

The STM system was built operating in air or in vacuum. A schematic diagram is shown in Fig. 1. Using PZT piezotubes (Quartz et Silice, PZT 160, Saint Pierre les Nemours, France) we can scan over the samples in a calibrated way [9]. An Olivetti PCS 86 computer monitored electrode voltages and stored the data. The feedback circuit cut-off frequency was about 3000 Hz. The scanning system is decoupled from outer vibrations by a two stage suspension: calibrated elastic in air or calibrated springs in UHV. The cut-off frequency was less than 1 Hz. Our microscopes are able to acquire a topographic signal in the constant current mode when tracking forward and simultaneously the spectroscopic signal when reversed during a line scan. For spectroscopic studies a sinusoidal voltage I’, < 0.1 I’, is superimposed on the d.c. bias voltage at a frequency well above the feedback cut-off of 10.3 kHz. The Z, component of the current is analyzed by a lock-in amplifier and gives the dZ,/d I’, representation in the (X, Y) plane. Except for a small amount of hysteresis this operating mode allows us to compare the two images directly. The tips were made of chemically etched tungsten wire (diameter 0.25 mm) in molar NaOH solution. All

T. Duoid et al. 1 Thin Solid Films 256 (1995) 268

)AC

269

272

3

CHI

sRd,npu, Anpul L---,--_-__,_____N : I

:

(a)

LINEARIZATION LOG

AM?

Fig. i_ Schematic diagram of STM indicating (full line) the arrangement for topographic studies and (dotted line) the specific part for spectroscopy.

images were taken with a positive tip bias and the tunnel current kept at 1 nA. Images were obtained via an AD converter with direct access to computer memory. The acquisition time for a 100 x 100 point image was about 100 s. All the images in this work are free from any analogue or digital filtering. Samples were obtained by thermal evaporation of silver on cleaved graphite (highly oriented pyrolytic graphite (HOPG), Union Carbide) at room temperature in a vacuum of 10-l Pa and at a deposition rate of 0.15 nm ss’.

-1

I

I

I

400

600

V(mv)l

)

(

0

200

800

(b)

Fig. 2. (a) Tunneling current versus voltage (I,- V,) for a tungsten tip and a polycrystalline Ag film. Curves I and 2 were obtained in air at two different places on the same sample under the same experimental conditions. (b) The normalized conductances corresponding to (a).

3. Results and discussion 3.1. Current - voltuge characteristics We began with a tunnel regime, Z, = 1 nA, I’, = 0.1 V, then the feedback was interrupted. The piezotube was extended to perform Z, = 0, I’, = 0, and at this unknown tip sample distance the I,- I’, characteristics were recorded at six different points on each film. The representative results are plotted in Figs. 2(a) and 3(a) for samples respectively studied in air and in a vacuum chamber after passage through the room atmosphere. The tunnelling resistance was always high (6-35 MW) indicating that we were far from the atomic contact as pointed out by Marchon et al. [lo] and that the Tersoff

and Hamann weak coupling approximation is relevant in our experimental conditions. However, curves recorded in air (Fig. 2(a)) show negative differential resistance (NDR) at 0.22 V, 0.5 V or 0.63 V while curves recorded in vacuum surroundings are always increasing. The corresponding ratios of differential to total conductivity are plotted versus V, on Figs. 2(b) and 3(b). As suggested by Feenstra et al. [ 1 l] this representation provides a good evaluation of the surface LDOS. In the energy range covered the silver LDOS presents only two weak variations near 0.15 and 0.3 V below the Fermi level [ 121. These features correspond to A and B

T. David et al. I Thin Solid Films 256 (1995) 268-272

270

25

1, (nA)

t

060

600

300

(4 (a)

I 100

Fig. 3. and a vacuum widths tances

I 200

I

I

I

300

400

500

, V(mV)

(b)

600

(a) Tunneling current versus voltage (I,- V,) for a tungsten tip polycrystalline Ag film. Curves I and 2 were obtained in at the same part of the sample but with two different gap before interrupting the feedback. (b) The normalized conduccorresponding to (a).

on Figs. 2(b) and 3(b). They are easy to locate in vacuum (Fig. 3(b)) but they are masked by the large decrease accompanying the NDR in air, We attribute this behaviour to atmospheric contamination. This contamination leads to inelastic processes in the junction associated with the atoms and molecules adsorbed on the surface. The silver surface reacts with sulfur and water molecules are also easily adsorbed on the tip and sample when working in air. A discrete set of electronic states appears in the tunnel junction: when electrons, moving from sample to tip are trapped on these levels Z, decreases and when traps are filled Z, increases again. This is in agreement with Garcia’s theoretical predic-

(cl Fig. 4. Unfiltered STM images of silver, particules on HOPG f, = I nA, VI = 0.25 V, 20 nm x 20 nm: (a) topography; (b) spectroscopy, VW = 0.025 V; (c) superposition of the two previous data showing the correlation between two islands only.

T. David et al. I Thin Solid Films 256 (1995) 268-272

tions [ 131 which connect NDR and localized surface barrier states. In vacuum, the sample is degassed for 72 h at a pressure lower than 10 ~’ Pa before study. We verify, in Fig. 3, that the surface is cleaner than in air: water molecules and sulphur are desorbed, the absolute value of Z, is lowered and NDR is absent. These results are coherent with the previous studies of Mamin et al. [ 141 on atmospheric contamination of the graphite-tungsten system and the observations of Tiedje et al. [ 151 and Tomanek et al. [ 161 of the I, variation with adsorption on tips.

?I1

Further work is running to clarify the role of such processes by recording I,- V, characteristics versus pressure and by varying the gas around the samples. 3.2. STM spectroscopy

on silver particles

md,film

In Fig. 4 we report topographic (Fig 4(a)) and spectroscopic (Fig. 4(b)) unfiltered data on silver islands obtained at V, = 0.3 V, V,. = 0.025 V and w = 10.32 kHz. The spectroscopic signal (acquired near area B of Figs. 2(b) and 3(b)) corresponds to the local variation of the density of states when the locally

(b. ii)

(b. i)

(b. iii)

Fig. 5. STM images of silver film, I, = 1 nA, 200 nm x 200 nm: (a) topography, V, = 0.45 V: maximum spectroscopy obtained with Vto = 0.025 and 0.45 V, 0.35 V and 0.25 V respectively as V,.

corrugation.

9.8 nm; (b) corresponding

212

T. David et al. I Thin Solid Films 256 (1995) 268-272

probed atom changes from carbon to silver. The signal is maximum at the particle perimeters and exceeds the corresponding topographic image areas indicating a near particle perturbation of the graphite LDOS by silver atoms. The absence of a spectroscopic signal at the location of one of the topographic features indicates that the tip is not probing silver in this area. Fig. 5 shows several images obtained in air on the film used for the I,- l’, curve labelled 2 in Fig. 2(a). In Fig. 5(a) we see a topographic image (V, = + 0.45) and Fig. 5(b) gives three spectroscopic views of the same area with three different bias modulation values. We therefore explore different zones in the sample LDOS. The spectroscopic response is important along the break appearing in the bottom part of the topography, especially for V, = 0.25 V. This energy is associated with NDR behaviour and the high spectroscopic signal indicates a concentration of electronic levels associated with the contamination in the rough areas of the film.

4. Conclusion The NDR behaviour observed only on I,- l’, characteristics recorded in air indicates that there is a strong interaction between a tunnel junction and the surrounding atoms. The corresponding enhancement of the spectroscopic signal offers the possibility of completing the information concerning the physical properties of the sample surface.

Acknowledgments The authors wish to acknowledge the Conseil R6 gional de Bourgogne and the Societe SPIRAL for their

financial support to the construction of the experimental set-up and Prof. J.P. Goudonnet for helpful discussion during this work.

References 111T.E.

Feuchtwang, Phys. Rev. B, lO( 1974) 4121,4135; Phys. Rec. B, 12 (1975) 3979; Phys. Rev. B, 13 (1976) 517. 121G. Binnig, H. Rohrer, C. Gerber and E. Weiber, Phys. Rev. Letf., 49 (1982) 57. [31 J. Bardeen, Phys. Rec. Left., 6 (1961) 57. [41 J. Tersoff and D.R. Hamann, Phys. Rev. B, 31 ( 1985) 805. J. Tersoff. Phys. Rev. B, 39 ( 1989) 1052; Phys. Rev. B, 40 ( 1989) 11980. [51 W. Sacks and C. Noguera, Phys. Rev. B, 43 (1991) 11612. [‘d J.B. Pendry, A.B. Pretre and B.C.H. Krutzen, J. Phys. Condens. Matter, 3 (1991) 4313, and references cited therein. [71 A.A. Lucas, J.P. Vigneron, Ph. Lambin, Th. Laloyaux and I. Derycke, Surf Sci., 269-270 (1992) 74. Rudiution EfJects [81 E. Louis, F. Flares and P.M. Echenique, Defecfs Solids, 109 (1989) 309. [91 J.P. Dufour, E. Bourillot and J.P. Goudonnet, J. Phys., I (1991) 1337. [lOI B. Marchon, M. Salmeron and N. Siekhaus, Phys. Rev. B, 39 ( 1989) 12907. []]I R.M. Feenstra, J.A. Stroscio and A.P. Fein, Surf: Sci., 181 (1987) 295; Phys. Rev. Lerr., 57 (1986) 2579. Handbook of the Band Structure oj [W D.A. Papaconstantopoulos, Elemental So/ids, Plenum, New York, 1976. N.V. Smith, Phys. Rev. B. 3 (1971) 1862. R. Garcia, J. Vuc. Sci. Technol. B, 9 (1991) 500. M.J. Mamin, E. Ganz, D.W. Abraham, R.E. Thomson and J. Clarke, Phys. Rev. B, 34 (1986) 9015. T. Tiedje, J. Varon, H. Deckman and J. Stokes, J. Vuc. Sci Technol. A, 6 (1988) 372. D. Tomanek, S.G. Louie, H.J. Mamin, D.W. Abraham, R.E. Thomson, E. Gane and J. Clarke, Phys. Rev. B, 35 (1987) 7790.