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Optically active heterostructures of graphene and ultrathin MoS2
Author's Accepted Manuscript
Optically active heterostructures of graphene and ultrathin MoS2 Kallol Roy, Medini Padmanabhan, Srijit Goswami, T. Phanindra Sai, Sanjeev Kaushal, Arindam Ghosh
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S0038-1098(13)00440-7 http://dx.doi.org/10.1016/j.ssc.2013.09.021 SSC12165
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Solid State Communications
Received date: 26 June 2013 Revised date: 17 September 2013 Accepted date: 19 September 2013 Cite this article as: Kallol Roy, Medini Padmanabhan, Srijit Goswami, T. Phanindra Sai, Sanjeev Kaushal, Arindam Ghosh, Optically active heterostructures of graphene and ultrathin MoS2, Solid State Communications, http://dx. doi.org/10.1016/j.ssc.2013.09.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Optically active heterostructures of graphene and ultrathin MoS2 Kallol Roy1 , Medini Padmanabhan1 , Srijit Goswami1∗ , T. Phanindra Sai1 , Sanjeev Kaushal2 , and Arindam Ghosh1 1
Department of Physics, Indian Institute of Science, Bangalore 560012, India and 2
Tokyo Electron Ltd., Akasaka Biz Tower,
3-1 Akasaka 5-Chome, Minato-ku, Tokyo 107-6325, Japan
Abstract Here we present the fabrication and characterisation of a new class of hybrid devices where the constituents are graphene and ultrathin molybdenum di-sulphide (MoS2 ). This device is one of the simplest member of a family of hybrids where the desirable electrical characteristics of graphene such as high mobility are combined with optical activity of semiconductors. We find that in the presence of an optically active substrate, considerable photoconductivity is induced in graphene which is persistent up to a time scale of at least several hours. This photo induced memory can be erased by the application of a suitable gate voltage pulse. This memory operation is stable for many cycles. We present a theoretical model based on localized states in MoS2 which explains the data.
Keywords: A. Graphene; A. Molybdenum disulfide; D. Photoconductivity; C. Heterostructure.
∗
Current address: Kavli Institute of Nanoscience, Delft University of Technology, P.O. Box 5046, 2600 GA Delft, The Netherlands.
1
I.
INTRODUCTION
Graphene is arguably the most researched layered material in the world today. The resounding success of graphene[1, 2] builds on its extremely high bipolar carrier mobility[3], band-structure dependent transport noise[4–7], as well as non-conventional information resources such as valleys[8]. Recently this has led researchers to expand their interests to explore possibility of ‘graphene analogues’ in the field of layered materials. For example, MoS2 with its optical sensitivity [9–11], and BN with its low trap density and high dielectric constant are being currently investigated for device applications[12, 13]. Combining two or more of such layered materials can give rise to interesting heterostructures where the desirable qualities of individual components can be combined to achieve novel device architectures. In this paper, we review our recent work on hybrid devices where the constituents are graphene and ultrathin MoS2 . We find that a considerable photoconductivity is induced in graphene in the presence of MoS2 and that the photoconductivity is persistent. We exploit this property of the system to demonstrate a memory device which has an optical ‘read’ and electrical ‘write’ feature. When combined with its superior electrical properties [5, 14], the optical properties of graphene such as strong coupling to light over wide range of wavelengths [15, 16] and fast recombination lifetimes [17, 18] make it a prominent candidate for optoelectronic devices. Ultra-fast photodetectors [19] have been demonstrated in graphene where the photocurrent is generated at a metal contact or a p − n junction. Two mechanisms, photovoltaic [20, 21] and photothermal [22–24], have been proposed to explain the generation of this photocurrent. However, an improvement in photoconductivity by many orders of magnitude has been achieved recently by combining graphene with other light absorbing materials such as quantum dots [25] and chromophores [26], which indicates that exotic optoelectronic response could be engineered in graphene based hybrids with an appropriate choice of complementary materials. Molybdenum disulphide (MoS2 ) is intrinsically responsive to light, owing to its bandgap which increases from about 1.2 eV (indirect) in bulk/multilayer MoS2 to 1.9 eV (direct) for a single molecular layer [27]. Along with luminescence [28] and negative photoconductivity [29], two dimensional (2D) sheets of MoS2 display desirable field-effect properties such as high on-off ratio and gain which are promising for device applications. While this makes
2
MoS2 electronically analogous to graphene, the nature of electronic states in MoS2 was found to be strongly localized [11], in sharp contrast to graphene, where charge carriers are inherently protected against localization. Consequently a 2D graphene-MoS2 hybrid emulates numerous doped semiconductor heterostructures and superlattices that display exotic optoelectronic effects, in particular, persistent photoconductivity (PPC).
II.
DESIGN OF GRAPHENE/MOS2 HYBRIDS
In fig. 1(a), we show a schematic of the transfer procedure by which a single-layer graphene flake is transferred onto a few-layer MoS2 flake. This procedure is similar to the one outlined in ref. [30]. Graphene is first exfoliated onto a glass slide covered with a transparent tape and a polymer layer. Separately, MoS2 is transferred to a Si/SiO2 substrate by standard mechanical exfoliation. The two are aligned under a microscope and brought into contact at an elevated temperature (∼ 100 ◦ C), which results in the transfer of graphene onto MoS2 . Once transferred onto MoS2 , graphene is etched into a desired shape by oxygen plasma. Contacts are drawn using standard e-beam lithography and metallization is done with Ti/Au (15 nm/45 nm) or Cr/Au (15 nm/45 nm). Measurements are done on graphene using a standard four-probe lock-in technique at a temperature of ∼ 100 ◦ C. Commercial white LED is used here as the light source. A schematic of the device is shown in fig. 1(b). Raman spectrum of graphene on MoS2 is shown in fig. 1(c) along with a single lorentzian fit [31].
III.
CONTROL DEVICES
We first present data from two control devices which clearly show that the hybrid architecture is necessary for the observation of non-trivial optoelectronic phenomena. In figure 2(a) we present data from a graphene flake exfoliated on Si/SiO2 substrate. The figure shows the four-probe resistance (R) as a function of gate voltage. Characteristic features of graphene including a Dirac point (VD ) close to Vg = 0 are observed. Note that both electron and hole dopings are achieved in our device on either side of VD . A scanning electron micrograph (SEM) of the device is shown in the inset.
3
The response of this device to white light (red vertical lines) and gate (blue vertical lines) pulses are shown in fig. 2(b). Note that there is no detectable response to the light pulse. This is expected since pristine graphene has a very short carrier recombination lifetime [17, 18] which results in negligible photoconductivity. Transient resistance-spikes in response to the gate pulses have been removed for clarity. To illustrate the transistor-like behavior of MoS2 , we present the variation of two-point conductance (G = ISD /VSD , the ratio of source-drain current to voltage) of MoS2 with Vg in fig. 2(c). This data is taken in a separate device with Au contacts on an MoS2 flake of comparable thickness with VSD = 0.1 V. Natural MoS2 is known to be a nominally n-type material. Recent studies in thin MoS2 flakes show that the electrical transport in these systems is dominated by the presence of localized states induced by the disorder potential due to the underlying SiO2 [11]. For Vg < Vt the Fermi level (EF ) of the system lies in the conduction band tail which results in very low conductivity. As Vg is raised above Vt , EF is pushed towards the extended states thereby causing conductivity to increase. The response of the bare MoS2 device to light and gate pulses is shown in fig. 2(d). Given that the bandgap of bulk MoS2 is 1.3 eV [9], we expect the device to exhibit photo> Vt this is indeed true, whereas for Vg < Vt there is conductivity [32]. We see that for Vg ∼ ∼ no detectable response to light. To understand this gate dependence of photoconductivity, we recall that for Vg < Vt , EF lies deep within the localized states which results in very low conductivity. Although illumination with white light results in raising the quasi-Fermi level of electrons, it is presumably still stuck within the localized states. Thus, the free electron carrier concentration in the conduction band is negligible even when the sample is illuminated. Therefore, no appreciable conductivity change occurs in our device for Vg < Vt . However, as Vg is raised to higher values, EF is pushed closer to the conduction band edge which consists of extended states. A slight increase in the quasi Fermi level (as a result of illumination) is now detectable as a conductivity change. Again, in fig. 2(d), the transient spikes in response to the pulsing of Vg have been removed for clarity.
IV.
INDUCED PHOTOCONDUCTIVITY IN GRAPHENE/MOS2 HYBRID
Figure 3(a) shows the four-probe resistance (R) of graphene on MoS2 as a function of gate voltage (Vg ). In the absence of light (black line) left side of the Dirac point shows 4
the usual R − Vg characteristics of graphene where as on the right side the effect of gate voltage becomes insignificant after some threshold value. This asymmetric response can be explained by considering the fact that MoS2 is known to act as a transistor [10, 11] itself. As Vg is swept from very negative to very positive voltages, the nature of MoS2 changes from dielectric to near-metallic. The value of Vg beyond which the underlying MoS2 transistor turns ‘on’ is denoted as Vt . Thus for Vg >> Vt , the metallic nature of MoS2 effectively screens Vg thereby causing the R − Vg curve of the overlying graphene to saturate beyond a certain value. The R − Vg curve in presence of light is shown by the solid red line in fig. 3(a). Note that there is a pronounced effect only on the negative side of the Dirac curve where an increase in resistance is observed in the presence of light. It can be inferred that some of the photo-generated carriers created inside MoS2 are transferred to graphene at negative gate biases. The gate voltage dependence of photoconductivity is further elucidated in fig. 3(b) where we show the change in resistance of the sample as a function of time. White light is periodically turned on and off as indicated by the vertical lines. Data is reported as the ratio of the resistance-change caused by illumination (∆R1 ) to the average value (Rmid ). We notice that appreciable photoconductivity is seen in the device only for Vg < Vt . In this regime, MoS2 acts like a dielectric, thereby sustaining a large in-built electric field. When light is signed on this device, electron hole pairs (EHP) which are generated inside MoS2 are swept in opposite directions by this field. The direction of the electric field is such that electrons are transferred to graphene and the holes are swept to the MoS2 /SiO2 interface. The addition of electrons to the already hole-doped graphene results in an increase in resistance of graphene as seen in our experiments. When Vg > Vt , MoS2 has a significant metallic character which results in negligible electric field in the bulk of MoS2 . When EHPs are generated as a result of illumination, a significant percentage of them recombine within the bulk before being transferred to graphene. We postulate that this is the reason for diminished photoconductivity for Vg > Vt . The difference in the band bending inside MoS2 which causes the asymmetry in the photoresponse of the system is schematically shown in the inset of fig. 3(a). Since the graphene work function (∼ 4.5 eV ) is lower than the MoS2 work function (∼ 5 eV ) we neglect the formation of Schottky barrier between graphene and MoS2 interface. Electric filed inside MoS2 because of gate electric field determines the charge transfer mechanism. 5
V.
PERSISTENT PHOTOCONDUCTIVITY
We would like to point out that the data shown in fig. 3(b) does not include the response of the system when light is turned on for the very first time. This is shown in fig. 4. Note that the initial dark resistance of the system (Ri ) is ∼ 0.93 kΩ. When light is turned on for the first time, the resistance of the system increases rapidly and saturates to a value given by Rl . Subsequent actions of turning light on an off results in a photoresponse similar to the one shown in fig. 3(b). However, note that, the system does not return to the initial dark state even after a long time. We denote this persistent state as Rp . Note that ∆R1 as defined in fig. 3(b) refers to Rl − Rp . This persistent photoconductivity (PPC) is indeed present in our samples over a wide range of gate voltages and is observed to be stable for several hours. In the subsequent sections, we demonstrate a memory/switching device which exploits the PPC in our system.
VI.
MEMORY EFFECT
In fig. 5(a), we illustrate the persistent photoconductivity (PPC) and an associated switching action in our device in greater details. We plot resistance as a function of time at a fixed gate voltage of -17.5 V. The application of a light pulse causes the resistance to spike. The value to which the resistance settles down is different from the initial value indicating the PPC in our system. The original state can however be recovered by the application of a gate pulse as shown (in this case, ∆V = 20V ). The resulting pulse sequence can be repeated many times recovering the persistent state resistance with 95% accuracy. The quantity ∆R2 is defined as the difference between these two states as shown in the figure. The logical operations which result in the toggling of the system between two resistance states are illustrated in fig. 5(a). Note that this effect is clearly observable only for gate voltages Vg ≪ Vt . At Vg > Vt , the system does not respond to a light pulse. In order to further elucidate the gate voltage dependence of the switching action, we show data taken at various Vg in fig. 5(b). In all the cases light pulse width, intensity, gate pulse width and gate pulse height have been kept constant. Note that the effect is almost nonexistent for Vg >-20 V. Similar experiments have been repeated with various light intensities (corresponding to
6
different currents through the LED as shown in fig. 6(a)) and the results are summarized in fig. 6(b) where ∆R2 /R is plotted as a function of Vg . We make the following observations: 1) at a given Vg , ∆R2 /R increases with increasing light intensity and saturates after a certain threshold, and 2) for a given light intensity ∆R2 /R increases with increasing magnitude of Vg . Thus, we show that the response of the device can be controlled by Vg and the intensity of light. In fig. 6(c) we show the effect of the magnitude of the gate pulse. Here, we attempt to retrieve the dark state of the system by gate pulses of varying magnitude as indicated in the figure. We find that initially a greater Vg pulse results in a better retrieval, although after a threshold (in this case ∆V = 20 V), the recovery is no longer sensitive to the pulse height. Another factor which determines the magnitude of ∆R2 is the duration of the light pulse as illustrated in fig. 6(d). The longer the duration of the light pulse, the greater is the value of ∆R2 .
VII.
THEORETICAL MODELING
A key feature of the switching cycles in Fig. 5a is the absence of a time-dependent decay in the photoconductivity in the persistent state, particularly at low photo-illumination intensity. In Fig. 7a we have plotted the photoconductivity in the persistent state in one of the cycles after a low-intensity pulse (ILED = 2 µA) for three different Vg . The PPC shows no decay with time over three decades irrespective of Vg , remaining essentially constant even when we monitored it over more than 10 hours (inset of Fig. 7b, here ILED = 5 µA and > 50 µA), we do observe a logarithmic decay Vg = −40V have been used). At higher ILED (∼ < 50 s, although at long times photoconductivity becomes nearly time-independent at times ∼ again. This is illustrated in Fig. 7b with PPC relaxation for ILED = 200 µA pulses. The near-absence of time decay of the persistent state suggests a strong potential barrier that prohibits recombination of electron and holes created on photo-illumination. To understand this we note that the majority carriers (electrons) in natural MoS2 flakes are strongly localized [11], and display Mott-type variable range hopping transport when Vg is reduced to the conduction threshold Vt . Thus our graphene-MoS2 hybrid behaves as a heterojunction of a doped conducting system (graphene) and a semiconductor (MoS2 ) where the carriers are localized, and trapped in potential wells that separate them from the conducting re7
gion [34]. Vt represents the gate voltage at which the Fermi energy (EF ) approaches the mobility edge (Ec ). Following ref. [34], the logarithmic decay in PPC can be explained with simple theoretical model. Consider a graphene sheet lying on a MoS2 substrate of thickness d. Time t = 0 denotes the moment when photoexcitation is turned off. The photogenerated carriers are swept by the electric field due to the applied gate voltage so that the electrons are transferred to graphene and the holes remain trapped in the MoS2 substrate. We assume a uniform volume density of trapped holes in the substrate given by Z. The instantaneous photo-electron density in graphene at time t is denoted by n(t). For t > 0, electron-hole recombination occurs. The electron-hole pairs which are spatially close recombine quickly while the farther ones recombine slowly leading to persistent photoconductivity. This is schematically indicated in fig. 7(c). Strong localization in MoS2 makes recombination via quantum tunneling or thermal activation virtually ineffective, and the equilibrium is restored only by lifting EF to EC , which provides the “erasing” mechanism of the PPC as R reverts to its dark-state resistance. We follow the theoretical treatment given by Queisser et al. [34] where the authors translate the spatial separation shown in fig. 7(c) into a time dependence, but generalize their analysis to include the effect of gate voltage. Our microscopic model relies on the fact that the excess number of electrons left behind in graphene at the Fermi energy EF is necessarily equal to the number of trapped holes in MoS2 at EF . This is proportional to the joint probability of two events: (1) quantum tunneling mediated electron-hole recombination, which results in the time dependent probability density p(x, t) of finding a trapped hole at distance x inside MoS2 from the MoS2 /graphene interface, and time t, and second, (2) probability of thermal excitation of a hole from the hole-doped graphene to MoS2 . The latter involves excitation across the energy barrier ∆E = −Ec −(−EF ) = EF −Ec , where the signs account for the excitation of a hole state, and Ec represents the mobility edge. The recombination rate of holes is given by a space-dependent recombination lifetime as: dp(x, t) −p(x, t) = . dt τ0 exp(2x/ξ)
(1)
Here τ0 represents the time constant for no spatial separation and ξ is a length scale of the order of the effective Bohr radius in MoS2 due to strong localization. Integrating, we get p(x, t) = p(x, 0) exp[−(t/τ0 ) exp(−2x/ξ)]. 8
(2)
Thus, the net excess electron density in graphene can be expressed as, ∫
n(t) = A exp [−∆E/kB T ]
d
p(x, t)dx 0
= A exp [(Ec − EF )/kB T ] ×
∫
d 0
p(x, 0) exp[−(t/τ0 ) exp(−2x/ξ)]dx.
(3)
where A is a gate voltage and time-independent normalization constant. We now adapt the sharp-front approximation given by Queisser et al. [34] valid for t >> τ0 which states that at a given t, all traps located before a particular distance xs are assumed to be neutralized. xs is defined by assuming that all carriers with lifetime t+τ0 have recombined at time t. That is, τ0 exp(2xs /ξ) = t + τ0 , which gives: xs (t) = (ξ/2)ln(t/τ0 )
f or t >> τ0 .
(4)
The integral in eqn. (3) can be simplified as: n(t) = A exp [(Ec − EF )/kB T ] = A exp [(Ec − EF )/kB T ]
∫
d
p(x, 0)dx xs ∫ d
p(x, 0)dx xs
= ZA(d − xs ) exp [(Ec − EF )/kB T ] = AZd(1 − (xs /d)) exp [(Ec − EF )/kB T ]. here we assume that p(x, 0) is a box distribution with height Z and width d. The expression for n(t) thus becomes, with a constant prefactor Z0 = AZ, n(t) = Z0 d f (t) exp[(EC − EF )/kB T ]
(5)
where f (t) = 1 − (ξ/2d) ln(t/τ0 ). Thus, the time dependence of the decay of photogenerated electron density, and hence that of the resistance (given that the change is resistance due to illumination is much smaller than that absolute value of resistance) is given by: R(t) = RA − RB ln(t/τ0 ), where RA and RB are time independent constants. 9
(6)
It is important to note that for higher LED intensity values f (t) deviates from logarithmic nature at large time and becomes nearly time independent (Fig. 7b). Such a nearly decay free ‘ON’ state is observed also when the intensity of LED pulse is low (Fig 7a). The ‘sharp front’ approximation is likely to break down at very long time and for weak illumination, as the distribution of trapped holes possibly becomes inhomogeneous. This can be viewed as formation of discrete hole pockets in MoS2 , so that p(x, 0) ∼ δ(x − d), where d is the distance from the interface. In such a limit, n(t) ∼ exp(−t/τ ), where τ = τ0 exp(2d/ξ). Since τ0 ∼ 10−9 sec, if we take d ∼ 10 nm (the order of MoS2 thickness) and ξ ∼ 0.5 nm (Bohr radius) we find τ ∼ 108 s, which indicates that lifetime of ‘ON’ state could exceed 2 − 5 years! At a given time t >> τ0 , the remnant photo-electron density in graphene, denoted by ∆ng , is given by ∆ng = no exp(−EF /kB T ).
(7)
Here no is a slowly varying function of t, assumed to be constant over experimental timescales. Note that this expression captures the gate voltage dependence of ∆ng . For hole-doped graphene, √ EF = −¯ hvF kF = −¯hvF πng √
= −¯hvF (π/e)Cef f |Vg − VD |
(8)
where vF and kF are the Fermi velocity and wave vector in graphene, ng is the total electron density in graphene, and Cef f is the effective capacitance of the system. To compare with our data, we define a quantity ∆Vg = (e/Cef f )∆ng . Thus, we have √
∆Vg = VC exp(β |Vg − VD |)
(9)
where VC and β are constants and are independent of Vg . √
In fig. 7(d), we plot ∆Vg vs.
|Vg − VD | for various light intensities which shows good
agreement with our model. Experimentally, we get β ≈ 1.1 which indicates that Cef f is about a factor of ∼ 5 smaller than the bare capacitance of 285 nm SiO2 that we used as gate dielectric. This can be readily attributed to the quantum capacitance of the MoS2 film which acts as a ‘leaky’ capacitor. Note that, if we consider this modified value of the capacitance, we find that the mobility of graphene in our sample is ∼ 12, 000 cm2 V−1 s−1 which is higher than that typically obtained for exfoliated graphene on bare SiO2 substrates. 10
VIII.
CONCLUSION
In conclusion, we fabricate graphene - MoS2 hybrid devices by transferring micron-sized flakes of single-layer-graphene onto micron-sized flakes of few-layer-MoS2 . We observed induced photoconductivity in graphene which is found to be persistent on a time scale of hours. The persistent state can be reverted back to the original dark state by the application of a voltage pulse on the underlying backgate. We illustrate an optical-write, electrical-read device based in the above properties. We also present a theoretical model based on localized electronic states in MoS2 which quantitatively explains our data.
IX.
ACKNOWLEDGEMENT
We are grateful to Prof. Raghavan, Materials Research Center, IISc for providing us with CVD graphene which was used in a later part of this study. We thank Department of Science & Technology and Tokyo Electron Limited for financial support.
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(a)
glass slide
I+ V+ V- I-
transparent tape graphene polymer
MoS2 SiO2
SiO2
SiO2 Si
SiO2
Si
graphene on SiO2
(b)
graphene MoS2
+
+
Si
graphene on MoS 2
V-
I-
counts (a.u.)
Si substrate
SiO2
Si
(c)
2730 2630 wavenumber (cm -1 )
V
I
FIG. 1: Device fabrication (a) Schematic of transfer process with corresponding optical images (scale bars are of 10 µm length). The doted line in the third optical image indicates the outline of graphene. (b) Schematic of the final device. (c) Raman spectrum of graphene on MoS2 . Only the 2D peak is shown with the single lorentzian fit.
14
bare graphene
bare MoS2
(a)
6
G ( x 10 -7 Ω -1)
2 µm
R (kΩ)
90
4
2
(c)
60
30
0 0 -30
-15
0 15 V g (V)
30
-60
-30V
7.20
(b)
0.23
-10V
0.36
30
60
6.40 5.60
-40V
4.80
-30V
2.00
0.34
10V
0.92 0.90
30V
1.91
R (MΩ)
R (kΩ)
0 V g (V)
(d)
0.25
-20V
1.80 0.149 0.145 0.132
20V
0.129
30V
0.121
1.89 18
-30
24
40V
0.119
30
50
t (minutes)
t (s)
100
FIG. 2: Vg dependent (a) resistance of graphene on SiO2 and (c) conductance of MoS2 on SiO2 . Response of (b) single layer graphene on Si/SiO2 and (d) thin layer MoS2 on Si/SiO2 to light and gate pulses at different gate voltages. The vertical red (dashed) and blue (dotted) lines in (b) and (d) are used to indicate the light and gate pulses, respectively. Note that, a few points representing the transient response to gate pulse have been removed for clarity.
15
(b)
(a) light dark
∆R1 / Rmid (%)
R (kΩ)
1.8
1.2 Vg < Vt
Vg > Vt
0.6
8
-30 V
-8 8
-20 V
-8 8
-10 V
-8 8
0V
-8 8
V g = +10 V
-8
-30
0 Vg (V)
30
0
6
12 18 24 time (minutes)
30
FIG. 3: (a) R − Vg curve for exfoliated single-layer graphene on MoS2 . Inset: Band bending inside MoS2 at two extreme gate voltages which results in different pathways for the optically generated EHPs. (b) Response of the device to illumination by white light. The red (solid) lines represent the moments in time when light is turned on. The green (dashed) lines correspond to light being turned off.
R (kΩ)
1.05
Rl
1.00 Rp
0.95 Ri
0
600
1200 t (s)
1800
FIG. 4: Response of the system to light. Note that there are three levels for the resistance here. The initial dark value (Ri ), a value under continuous illumination (Rl ) and an intermediate persistent state which is attained when light is turned off (Rp ).
16
SET
RESET
(a) pulse sequence
light pulse gate voltage pulse
1.69
Vg= -17.5 V
0.92
∆R 2
R (kΩ)
R (kΩ)
Rp Vg = 5 V Rd
1.64
0.87 0
200
400
600
t (s)
(b)
-20 V
1.59 1.57
-25 V
1.13
R (kΩ)
1.11 -30 V
0.83 0.81
-35 V
0.64 0.62
-40 V
0.52 0.50
10
20
30
40
50
t (minutes)
FIG. 5: (a) Toggling of the system between two states. R vs t plot is shown illustrating the ‘set’ and ‘reset’ operations achieved by light and gate pulses, respectively. For both curves, ILED = 5 mA. Note that the transient spikes in R following the gate pulses have been removed throughout the manuscript. The vertical red (dashed) and green (dotted) lines are used to indicate the light and gate pulses, respectively. (b) Evolution of the switching action as function of Vg .
17
20 µA 2 µA
2000µA 200 µA
ILED
ILED
3
0.53
0.50
(a)
4
12 t (minutes)
20
-30 Vg (V)
-20 0.8
Rl
0.7
∆V = 5 V
}
Rp
10 V
∆R 2 Rd
0
}
(c)
}
0.84
R (kΩ)
∆R 1
}
R (kΩ)
-40
(d)
0.88 0.86
1
(b)
Vg = -40 V
∆R 2 /Rd(%)
R (kΩ)
0.56
5 2 µA 20 µA 200 µA 2000µA
20 V
30 V
1000 2000 t (s)
3000
0
400 t (s)
0.6
800
FIG. 6: (a) Switching action for four different light intensities at a given Vg (b) Percentage change in resistance as a function of Vg for various illumination intensities. The values of current flowing through the illumination LED are indicated. (c) Effect of the gate pulse height on the dark state recovery. (b) Effect of the width of the light pulse on the switching effect.
18
8
26
-30 V
2
R - R (Ω)
R - R (Ω) d
(b) 8
0
R - R (Ω)
(a)
d
-35 V
2 8
t (s) -40 V
0.1
1
10
-30 V
14
-40 V
2
100
1
10
100
t (s)
t (s)
(c)
1E2 1E3 1E4
d
8
Graphene
MoS2
p+ Si
SiO2
DOS Ec EF
localized states x
Vg
t=0 trapped holes
electrons
t>>τ 0 (persistent regime)
(d) 1
ILED
∆Vg (V)
2000µA 200 µA 20 µA 2 µA
R
∆R
R
0.1
∆Vg
time
3
4 (Vg - VD )
1/2
5 (V
1/2
Vg
)
FIG. 7: (a) Nearly relaxation free nature of persistent states at three different gate voltages. ILED = 2 µA has been used for 0.1 s to ‘excite’ the system. (b) Transition from logarithmic decay to nearly relaxation free state after photoexcitation with LED current 200 µA for 0.1s. Solid lines are guide to eye. Inset shows the trace of ‘ON’ state over long time scale (∼ 12 hrs). (c) Schematic showing the evolution of electron-hole distribution in the system as a function of time. t = 0 corresponds to the moment after the light pulse. (d) ∆Vg vs.
19
√
Vg − VD . This plot is
calculated using the data shown in Fig. 5(b). The inset illustrates the conversion of ∆R to ∆Vg . Note: Fig. a, b, c, and partially d are adapted from ref. [33].
Optically active heterostructures of graphene and ultrathin MoS2 Kallol Roy, Medini Padmanabhan, Srijit Goswami, T. Phanindra Sai, Sanjeev Kaushal, Arindam Ghosh
www.elsevier.com/locate/ssc
PII: DOI: Reference:
S0038-1098(13)00440-7 http://dx.doi.org/10.1016/j.ssc.2013.09.021 SSC12165
To appear in:
Solid State Communications
Received date: 26 June 2013 Revised date: 17 September 2013 Accepted date: 19 September 2013 Cite this article as: Kallol Roy, Medini Padmanabhan, Srijit Goswami, T. Phanindra Sai, Sanjeev Kaushal, Arindam Ghosh, Optically active heterostructures of graphene and ultrathin MoS2, Solid State Communications, http://dx. doi.org/10.1016/j.ssc.2013.09.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Optically active heterostructures of graphene and ultrathin MoS2 Kallol Roy1 , Medini Padmanabhan1 , Srijit Goswami1∗ , T. Phanindra Sai1 , Sanjeev Kaushal2 , and Arindam Ghosh1 1
Department of Physics, Indian Institute of Science, Bangalore 560012, India and 2
Tokyo Electron Ltd., Akasaka Biz Tower,
3-1 Akasaka 5-Chome, Minato-ku, Tokyo 107-6325, Japan
Abstract Here we present the fabrication and characterisation of a new class of hybrid devices where the constituents are graphene and ultrathin molybdenum di-sulphide (MoS2 ). This device is one of the simplest member of a family of hybrids where the desirable electrical characteristics of graphene such as high mobility are combined with optical activity of semiconductors. We find that in the presence of an optically active substrate, considerable photoconductivity is induced in graphene which is persistent up to a time scale of at least several hours. This photo induced memory can be erased by the application of a suitable gate voltage pulse. This memory operation is stable for many cycles. We present a theoretical model based on localized states in MoS2 which explains the data.
Keywords: A. Graphene; A. Molybdenum disulfide; D. Photoconductivity; C. Heterostructure.
∗
Current address: Kavli Institute of Nanoscience, Delft University of Technology, P.O. Box 5046, 2600 GA Delft, The Netherlands.
1
I.
INTRODUCTION
Graphene is arguably the most researched layered material in the world today. The resounding success of graphene[1, 2] builds on its extremely high bipolar carrier mobility[3], band-structure dependent transport noise[4–7], as well as non-conventional information resources such as valleys[8]. Recently this has led researchers to expand their interests to explore possibility of ‘graphene analogues’ in the field of layered materials. For example, MoS2 with its optical sensitivity [9–11], and BN with its low trap density and high dielectric constant are being currently investigated for device applications[12, 13]. Combining two or more of such layered materials can give rise to interesting heterostructures where the desirable qualities of individual components can be combined to achieve novel device architectures. In this paper, we review our recent work on hybrid devices where the constituents are graphene and ultrathin MoS2 . We find that a considerable photoconductivity is induced in graphene in the presence of MoS2 and that the photoconductivity is persistent. We exploit this property of the system to demonstrate a memory device which has an optical ‘read’ and electrical ‘write’ feature. When combined with its superior electrical properties [5, 14], the optical properties of graphene such as strong coupling to light over wide range of wavelengths [15, 16] and fast recombination lifetimes [17, 18] make it a prominent candidate for optoelectronic devices. Ultra-fast photodetectors [19] have been demonstrated in graphene where the photocurrent is generated at a metal contact or a p − n junction. Two mechanisms, photovoltaic [20, 21] and photothermal [22–24], have been proposed to explain the generation of this photocurrent. However, an improvement in photoconductivity by many orders of magnitude has been achieved recently by combining graphene with other light absorbing materials such as quantum dots [25] and chromophores [26], which indicates that exotic optoelectronic response could be engineered in graphene based hybrids with an appropriate choice of complementary materials. Molybdenum disulphide (MoS2 ) is intrinsically responsive to light, owing to its bandgap which increases from about 1.2 eV (indirect) in bulk/multilayer MoS2 to 1.9 eV (direct) for a single molecular layer [27]. Along with luminescence [28] and negative photoconductivity [29], two dimensional (2D) sheets of MoS2 display desirable field-effect properties such as high on-off ratio and gain which are promising for device applications. While this makes
2
MoS2 electronically analogous to graphene, the nature of electronic states in MoS2 was found to be strongly localized [11], in sharp contrast to graphene, where charge carriers are inherently protected against localization. Consequently a 2D graphene-MoS2 hybrid emulates numerous doped semiconductor heterostructures and superlattices that display exotic optoelectronic effects, in particular, persistent photoconductivity (PPC).
II.
DESIGN OF GRAPHENE/MOS2 HYBRIDS
In fig. 1(a), we show a schematic of the transfer procedure by which a single-layer graphene flake is transferred onto a few-layer MoS2 flake. This procedure is similar to the one outlined in ref. [30]. Graphene is first exfoliated onto a glass slide covered with a transparent tape and a polymer layer. Separately, MoS2 is transferred to a Si/SiO2 substrate by standard mechanical exfoliation. The two are aligned under a microscope and brought into contact at an elevated temperature (∼ 100 ◦ C), which results in the transfer of graphene onto MoS2 . Once transferred onto MoS2 , graphene is etched into a desired shape by oxygen plasma. Contacts are drawn using standard e-beam lithography and metallization is done with Ti/Au (15 nm/45 nm) or Cr/Au (15 nm/45 nm). Measurements are done on graphene using a standard four-probe lock-in technique at a temperature of ∼ 100 ◦ C. Commercial white LED is used here as the light source. A schematic of the device is shown in fig. 1(b). Raman spectrum of graphene on MoS2 is shown in fig. 1(c) along with a single lorentzian fit [31].
III.
CONTROL DEVICES
We first present data from two control devices which clearly show that the hybrid architecture is necessary for the observation of non-trivial optoelectronic phenomena. In figure 2(a) we present data from a graphene flake exfoliated on Si/SiO2 substrate. The figure shows the four-probe resistance (R) as a function of gate voltage. Characteristic features of graphene including a Dirac point (VD ) close to Vg = 0 are observed. Note that both electron and hole dopings are achieved in our device on either side of VD . A scanning electron micrograph (SEM) of the device is shown in the inset.
3
The response of this device to white light (red vertical lines) and gate (blue vertical lines) pulses are shown in fig. 2(b). Note that there is no detectable response to the light pulse. This is expected since pristine graphene has a very short carrier recombination lifetime [17, 18] which results in negligible photoconductivity. Transient resistance-spikes in response to the gate pulses have been removed for clarity. To illustrate the transistor-like behavior of MoS2 , we present the variation of two-point conductance (G = ISD /VSD , the ratio of source-drain current to voltage) of MoS2 with Vg in fig. 2(c). This data is taken in a separate device with Au contacts on an MoS2 flake of comparable thickness with VSD = 0.1 V. Natural MoS2 is known to be a nominally n-type material. Recent studies in thin MoS2 flakes show that the electrical transport in these systems is dominated by the presence of localized states induced by the disorder potential due to the underlying SiO2 [11]. For Vg < Vt the Fermi level (EF ) of the system lies in the conduction band tail which results in very low conductivity. As Vg is raised above Vt , EF is pushed towards the extended states thereby causing conductivity to increase. The response of the bare MoS2 device to light and gate pulses is shown in fig. 2(d). Given that the bandgap of bulk MoS2 is 1.3 eV [9], we expect the device to exhibit photo> Vt this is indeed true, whereas for Vg < Vt there is conductivity [32]. We see that for Vg ∼ ∼ no detectable response to light. To understand this gate dependence of photoconductivity, we recall that for Vg < Vt , EF lies deep within the localized states which results in very low conductivity. Although illumination with white light results in raising the quasi-Fermi level of electrons, it is presumably still stuck within the localized states. Thus, the free electron carrier concentration in the conduction band is negligible even when the sample is illuminated. Therefore, no appreciable conductivity change occurs in our device for Vg < Vt . However, as Vg is raised to higher values, EF is pushed closer to the conduction band edge which consists of extended states. A slight increase in the quasi Fermi level (as a result of illumination) is now detectable as a conductivity change. Again, in fig. 2(d), the transient spikes in response to the pulsing of Vg have been removed for clarity.
IV.
INDUCED PHOTOCONDUCTIVITY IN GRAPHENE/MOS2 HYBRID
Figure 3(a) shows the four-probe resistance (R) of graphene on MoS2 as a function of gate voltage (Vg ). In the absence of light (black line) left side of the Dirac point shows 4
the usual R − Vg characteristics of graphene where as on the right side the effect of gate voltage becomes insignificant after some threshold value. This asymmetric response can be explained by considering the fact that MoS2 is known to act as a transistor [10, 11] itself. As Vg is swept from very negative to very positive voltages, the nature of MoS2 changes from dielectric to near-metallic. The value of Vg beyond which the underlying MoS2 transistor turns ‘on’ is denoted as Vt . Thus for Vg >> Vt , the metallic nature of MoS2 effectively screens Vg thereby causing the R − Vg curve of the overlying graphene to saturate beyond a certain value. The R − Vg curve in presence of light is shown by the solid red line in fig. 3(a). Note that there is a pronounced effect only on the negative side of the Dirac curve where an increase in resistance is observed in the presence of light. It can be inferred that some of the photo-generated carriers created inside MoS2 are transferred to graphene at negative gate biases. The gate voltage dependence of photoconductivity is further elucidated in fig. 3(b) where we show the change in resistance of the sample as a function of time. White light is periodically turned on and off as indicated by the vertical lines. Data is reported as the ratio of the resistance-change caused by illumination (∆R1 ) to the average value (Rmid ). We notice that appreciable photoconductivity is seen in the device only for Vg < Vt . In this regime, MoS2 acts like a dielectric, thereby sustaining a large in-built electric field. When light is signed on this device, electron hole pairs (EHP) which are generated inside MoS2 are swept in opposite directions by this field. The direction of the electric field is such that electrons are transferred to graphene and the holes are swept to the MoS2 /SiO2 interface. The addition of electrons to the already hole-doped graphene results in an increase in resistance of graphene as seen in our experiments. When Vg > Vt , MoS2 has a significant metallic character which results in negligible electric field in the bulk of MoS2 . When EHPs are generated as a result of illumination, a significant percentage of them recombine within the bulk before being transferred to graphene. We postulate that this is the reason for diminished photoconductivity for Vg > Vt . The difference in the band bending inside MoS2 which causes the asymmetry in the photoresponse of the system is schematically shown in the inset of fig. 3(a). Since the graphene work function (∼ 4.5 eV ) is lower than the MoS2 work function (∼ 5 eV ) we neglect the formation of Schottky barrier between graphene and MoS2 interface. Electric filed inside MoS2 because of gate electric field determines the charge transfer mechanism. 5
V.
PERSISTENT PHOTOCONDUCTIVITY
We would like to point out that the data shown in fig. 3(b) does not include the response of the system when light is turned on for the very first time. This is shown in fig. 4. Note that the initial dark resistance of the system (Ri ) is ∼ 0.93 kΩ. When light is turned on for the first time, the resistance of the system increases rapidly and saturates to a value given by Rl . Subsequent actions of turning light on an off results in a photoresponse similar to the one shown in fig. 3(b). However, note that, the system does not return to the initial dark state even after a long time. We denote this persistent state as Rp . Note that ∆R1 as defined in fig. 3(b) refers to Rl − Rp . This persistent photoconductivity (PPC) is indeed present in our samples over a wide range of gate voltages and is observed to be stable for several hours. In the subsequent sections, we demonstrate a memory/switching device which exploits the PPC in our system.
VI.
MEMORY EFFECT
In fig. 5(a), we illustrate the persistent photoconductivity (PPC) and an associated switching action in our device in greater details. We plot resistance as a function of time at a fixed gate voltage of -17.5 V. The application of a light pulse causes the resistance to spike. The value to which the resistance settles down is different from the initial value indicating the PPC in our system. The original state can however be recovered by the application of a gate pulse as shown (in this case, ∆V = 20V ). The resulting pulse sequence can be repeated many times recovering the persistent state resistance with 95% accuracy. The quantity ∆R2 is defined as the difference between these two states as shown in the figure. The logical operations which result in the toggling of the system between two resistance states are illustrated in fig. 5(a). Note that this effect is clearly observable only for gate voltages Vg ≪ Vt . At Vg > Vt , the system does not respond to a light pulse. In order to further elucidate the gate voltage dependence of the switching action, we show data taken at various Vg in fig. 5(b). In all the cases light pulse width, intensity, gate pulse width and gate pulse height have been kept constant. Note that the effect is almost nonexistent for Vg >-20 V. Similar experiments have been repeated with various light intensities (corresponding to
6
different currents through the LED as shown in fig. 6(a)) and the results are summarized in fig. 6(b) where ∆R2 /R is plotted as a function of Vg . We make the following observations: 1) at a given Vg , ∆R2 /R increases with increasing light intensity and saturates after a certain threshold, and 2) for a given light intensity ∆R2 /R increases with increasing magnitude of Vg . Thus, we show that the response of the device can be controlled by Vg and the intensity of light. In fig. 6(c) we show the effect of the magnitude of the gate pulse. Here, we attempt to retrieve the dark state of the system by gate pulses of varying magnitude as indicated in the figure. We find that initially a greater Vg pulse results in a better retrieval, although after a threshold (in this case ∆V = 20 V), the recovery is no longer sensitive to the pulse height. Another factor which determines the magnitude of ∆R2 is the duration of the light pulse as illustrated in fig. 6(d). The longer the duration of the light pulse, the greater is the value of ∆R2 .
VII.
THEORETICAL MODELING
A key feature of the switching cycles in Fig. 5a is the absence of a time-dependent decay in the photoconductivity in the persistent state, particularly at low photo-illumination intensity. In Fig. 7a we have plotted the photoconductivity in the persistent state in one of the cycles after a low-intensity pulse (ILED = 2 µA) for three different Vg . The PPC shows no decay with time over three decades irrespective of Vg , remaining essentially constant even when we monitored it over more than 10 hours (inset of Fig. 7b, here ILED = 5 µA and > 50 µA), we do observe a logarithmic decay Vg = −40V have been used). At higher ILED (∼ < 50 s, although at long times photoconductivity becomes nearly time-independent at times ∼ again. This is illustrated in Fig. 7b with PPC relaxation for ILED = 200 µA pulses. The near-absence of time decay of the persistent state suggests a strong potential barrier that prohibits recombination of electron and holes created on photo-illumination. To understand this we note that the majority carriers (electrons) in natural MoS2 flakes are strongly localized [11], and display Mott-type variable range hopping transport when Vg is reduced to the conduction threshold Vt . Thus our graphene-MoS2 hybrid behaves as a heterojunction of a doped conducting system (graphene) and a semiconductor (MoS2 ) where the carriers are localized, and trapped in potential wells that separate them from the conducting re7
gion [34]. Vt represents the gate voltage at which the Fermi energy (EF ) approaches the mobility edge (Ec ). Following ref. [34], the logarithmic decay in PPC can be explained with simple theoretical model. Consider a graphene sheet lying on a MoS2 substrate of thickness d. Time t = 0 denotes the moment when photoexcitation is turned off. The photogenerated carriers are swept by the electric field due to the applied gate voltage so that the electrons are transferred to graphene and the holes remain trapped in the MoS2 substrate. We assume a uniform volume density of trapped holes in the substrate given by Z. The instantaneous photo-electron density in graphene at time t is denoted by n(t). For t > 0, electron-hole recombination occurs. The electron-hole pairs which are spatially close recombine quickly while the farther ones recombine slowly leading to persistent photoconductivity. This is schematically indicated in fig. 7(c). Strong localization in MoS2 makes recombination via quantum tunneling or thermal activation virtually ineffective, and the equilibrium is restored only by lifting EF to EC , which provides the “erasing” mechanism of the PPC as R reverts to its dark-state resistance. We follow the theoretical treatment given by Queisser et al. [34] where the authors translate the spatial separation shown in fig. 7(c) into a time dependence, but generalize their analysis to include the effect of gate voltage. Our microscopic model relies on the fact that the excess number of electrons left behind in graphene at the Fermi energy EF is necessarily equal to the number of trapped holes in MoS2 at EF . This is proportional to the joint probability of two events: (1) quantum tunneling mediated electron-hole recombination, which results in the time dependent probability density p(x, t) of finding a trapped hole at distance x inside MoS2 from the MoS2 /graphene interface, and time t, and second, (2) probability of thermal excitation of a hole from the hole-doped graphene to MoS2 . The latter involves excitation across the energy barrier ∆E = −Ec −(−EF ) = EF −Ec , where the signs account for the excitation of a hole state, and Ec represents the mobility edge. The recombination rate of holes is given by a space-dependent recombination lifetime as: dp(x, t) −p(x, t) = . dt τ0 exp(2x/ξ)
(1)
Here τ0 represents the time constant for no spatial separation and ξ is a length scale of the order of the effective Bohr radius in MoS2 due to strong localization. Integrating, we get p(x, t) = p(x, 0) exp[−(t/τ0 ) exp(−2x/ξ)]. 8
(2)
Thus, the net excess electron density in graphene can be expressed as, ∫
n(t) = A exp [−∆E/kB T ]
d
p(x, t)dx 0
= A exp [(Ec − EF )/kB T ] ×
∫
d 0
p(x, 0) exp[−(t/τ0 ) exp(−2x/ξ)]dx.
(3)
where A is a gate voltage and time-independent normalization constant. We now adapt the sharp-front approximation given by Queisser et al. [34] valid for t >> τ0 which states that at a given t, all traps located before a particular distance xs are assumed to be neutralized. xs is defined by assuming that all carriers with lifetime t+τ0 have recombined at time t. That is, τ0 exp(2xs /ξ) = t + τ0 , which gives: xs (t) = (ξ/2)ln(t/τ0 )
f or t >> τ0 .
(4)
The integral in eqn. (3) can be simplified as: n(t) = A exp [(Ec − EF )/kB T ] = A exp [(Ec − EF )/kB T ]
∫
d
p(x, 0)dx xs ∫ d
p(x, 0)dx xs
= ZA(d − xs ) exp [(Ec − EF )/kB T ] = AZd(1 − (xs /d)) exp [(Ec − EF )/kB T ]. here we assume that p(x, 0) is a box distribution with height Z and width d. The expression for n(t) thus becomes, with a constant prefactor Z0 = AZ, n(t) = Z0 d f (t) exp[(EC − EF )/kB T ]
(5)
where f (t) = 1 − (ξ/2d) ln(t/τ0 ). Thus, the time dependence of the decay of photogenerated electron density, and hence that of the resistance (given that the change is resistance due to illumination is much smaller than that absolute value of resistance) is given by: R(t) = RA − RB ln(t/τ0 ), where RA and RB are time independent constants. 9
(6)
It is important to note that for higher LED intensity values f (t) deviates from logarithmic nature at large time and becomes nearly time independent (Fig. 7b). Such a nearly decay free ‘ON’ state is observed also when the intensity of LED pulse is low (Fig 7a). The ‘sharp front’ approximation is likely to break down at very long time and for weak illumination, as the distribution of trapped holes possibly becomes inhomogeneous. This can be viewed as formation of discrete hole pockets in MoS2 , so that p(x, 0) ∼ δ(x − d), where d is the distance from the interface. In such a limit, n(t) ∼ exp(−t/τ ), where τ = τ0 exp(2d/ξ). Since τ0 ∼ 10−9 sec, if we take d ∼ 10 nm (the order of MoS2 thickness) and ξ ∼ 0.5 nm (Bohr radius) we find τ ∼ 108 s, which indicates that lifetime of ‘ON’ state could exceed 2 − 5 years! At a given time t >> τ0 , the remnant photo-electron density in graphene, denoted by ∆ng , is given by ∆ng = no exp(−EF /kB T ).
(7)
Here no is a slowly varying function of t, assumed to be constant over experimental timescales. Note that this expression captures the gate voltage dependence of ∆ng . For hole-doped graphene, √ EF = −¯ hvF kF = −¯hvF πng √
= −¯hvF (π/e)Cef f |Vg − VD |
(8)
where vF and kF are the Fermi velocity and wave vector in graphene, ng is the total electron density in graphene, and Cef f is the effective capacitance of the system. To compare with our data, we define a quantity ∆Vg = (e/Cef f )∆ng . Thus, we have √
∆Vg = VC exp(β |Vg − VD |)
(9)
where VC and β are constants and are independent of Vg . √
In fig. 7(d), we plot ∆Vg vs.
|Vg − VD | for various light intensities which shows good
agreement with our model. Experimentally, we get β ≈ 1.1 which indicates that Cef f is about a factor of ∼ 5 smaller than the bare capacitance of 285 nm SiO2 that we used as gate dielectric. This can be readily attributed to the quantum capacitance of the MoS2 film which acts as a ‘leaky’ capacitor. Note that, if we consider this modified value of the capacitance, we find that the mobility of graphene in our sample is ∼ 12, 000 cm2 V−1 s−1 which is higher than that typically obtained for exfoliated graphene on bare SiO2 substrates. 10
VIII.
CONCLUSION
In conclusion, we fabricate graphene - MoS2 hybrid devices by transferring micron-sized flakes of single-layer-graphene onto micron-sized flakes of few-layer-MoS2 . We observed induced photoconductivity in graphene which is found to be persistent on a time scale of hours. The persistent state can be reverted back to the original dark state by the application of a voltage pulse on the underlying backgate. We illustrate an optical-write, electrical-read device based in the above properties. We also present a theoretical model based on localized electronic states in MoS2 which quantitatively explains our data.
IX.
ACKNOWLEDGEMENT
We are grateful to Prof. Raghavan, Materials Research Center, IISc for providing us with CVD graphene which was used in a later part of this study. We thank Department of Science & Technology and Tokyo Electron Limited for financial support.
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[9] Mak, K. F., Lee, C., Hone, J., Shan, J. & Heinz, T. F. Atomically thin MoS2 : A new direct-gap semiconductor. Phys. Rev. Lett. 105, 136805 (2010). [10] Radisavljevic, B., Radenovic, A., Brivio, J., Giacometti, V. & Kis, A. Single-layer MoS2 transistors. Nature Nanotech. 6, 147150 (2011). [11] Ghatak, S., Pal, A. N. & Ghosh, A. Nature of electronic states in atomically thin MoS2 field-effect transistors. ACS Nano 5, 7707–7712 (2011). [12] Jin, C., Lin, F., Suenaga, K. & Iijima, S. Fabrication of a freestanding boron nitride single layer and its defect assignments. Phys. Rev. Lett. 102, 195505 (2009). [13] Dean., C. et al. Boron nitride substrates for high-quality graphene electronics. Nature Nanotech. 5, 722–726 (2010). [14] Das Sarma, S., Adam, S., Hwang, E. H. & Rossi, E. Electronic transport in two-dimensional graphene. Rev. Mod. Phys. 83, 407–470 (2011). [15] Dawlaty, J. M. et al. Measurement of the optical absorption spectra of epitaxial graphene from terahertz to visible. Appl. Phys. Lett. 93, 131905 (2008). [16] Nair, R. R. et al. Fine structure constant defines visual transparency of graphene. Science 320, 1308 (2008). [17] George, P. A. et al. Ultrafast optical-pump terahertz-probe spectroscopy of the carrier relaxation and recombination dynamics in epitaxial graphene. Nano Lett. 8, 4248–4251 (2008). [18] Urich, A., Unterrainer, K. & Mueller, T. Intrinsic response time of graphene photodetectors. Nano Lett. 11, 2804–2808 (2011). [19] Xia, F., Mueller, T., Lin, Y.-m., Valdes-Garcia, A. & Avouris, P. Ultrafast graphene photodetector. Nature Nanotech. 4, 839–843 (2009). [20] Xia, F. et al. Photocurrent imaging and efficient photon detection in a graphene transistor. Nano Lett. 9, 1039–1044 (2009). [21] Ghosh, S., Sarker, B. K., Chunder, A., Zhai, L. & Khondaker, S. I. Position dependent photodetector from large area reduced graphene oxide thin films. Appl. Phys. Lett. 96, 163109 (2010). [22] Xu, X., Gabor, N. M., Alden, J. S., van der Zande, A. M. & McEuen, P. L.
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[24] Gabor, N. M. et al. Hot carrier-assisted intrinsic photoresponse in graphene. Science 334, 648–652 (2011). [25] Konstantatos, G. et al. Hybrid graphene-quantum dot phototransistors with ultrahigh gain. Nature Nanotech. 7, 363–368 (2012). [26] Kim, M., Safron, N. S., Huang, C., Arnold, M. S. & Gopalan, P. Light-driven reversible modulation of doping in graphene. Nano Lett. 12, 182–187 (2012). [27] Mak, K. F., Lee, C., Hone, J., Shan, J. & Heinz, T. F. Atomically thin MoS2 : A new direct-gap semiconductor. Phys. Rev. Lett. 105, 136805 (2010). [28] Splendiani, A. et al. Emerging photoluminescence in monolayer MoS2 . Nano Lett. 10, 1271– 1275 (2010). [29] Serpi, A. Negative photoconductivity in MoS2 . Phys. Stat. Sol. (a) 133, K73–K77 (1992). [30] Zomer, P. J., Dash, S. P., Tombros, N. & van Wees, B. J. A transfer technique for high mobility graphene devices on commercially available hexagonal boron nitride. Appl. Phys. Lett. 99, 232104 (2011). [31] Ferrari, A. C. et al. Raman spectrum of graphene and graphene layers. Phys. Rev. Lett. 97, 187401 (2006). [32] Lopez-Sanchez, O., Lembke, D., Kayci, M., Radenovic, A. & Kis, A. Ultrasensitive photodetectors based on monolayer MoS2 . Nat Nano (2013). [33] Roy, K. et al. Multifunctional graphene-MoS2 hybrid for colossal photoresponsivity and relaxation-free opto-electronic memory. Under review . [34] Queisser, H. J. & Theodorou, D. E. Decay kinetics of persistent photoconductivity in semiconductors. Phys. Rev. B 33, 4027–4033 (1986).
13
(a)
glass slide
I+ V+ V- I-
transparent tape graphene polymer
MoS2 SiO2
SiO2
SiO2 Si
SiO2
Si
graphene on SiO2
(b)
graphene MoS2
+
+
Si
graphene on MoS 2
V-
I-
counts (a.u.)
Si substrate
SiO2
Si
(c)
2730 2630 wavenumber (cm -1 )
V
I
FIG. 1: Device fabrication (a) Schematic of transfer process with corresponding optical images (scale bars are of 10 µm length). The doted line in the third optical image indicates the outline of graphene. (b) Schematic of the final device. (c) Raman spectrum of graphene on MoS2 . Only the 2D peak is shown with the single lorentzian fit.
14
bare graphene
bare MoS2
(a)
6
G ( x 10 -7 Ω -1)
2 µm
R (kΩ)
90
4
2
(c)
60
30
0 0 -30
-15
0 15 V g (V)
30
-60
-30V
7.20
(b)
0.23
-10V
0.36
30
60
6.40 5.60
-40V
4.80
-30V
2.00
0.34
10V
0.92 0.90
30V
1.91
R (MΩ)
R (kΩ)
0 V g (V)
(d)
0.25
-20V
1.80 0.149 0.145 0.132
20V
0.129
30V
0.121
1.89 18
-30
24
40V
0.119
30
50
t (minutes)
t (s)
100
FIG. 2: Vg dependent (a) resistance of graphene on SiO2 and (c) conductance of MoS2 on SiO2 . Response of (b) single layer graphene on Si/SiO2 and (d) thin layer MoS2 on Si/SiO2 to light and gate pulses at different gate voltages. The vertical red (dashed) and blue (dotted) lines in (b) and (d) are used to indicate the light and gate pulses, respectively. Note that, a few points representing the transient response to gate pulse have been removed for clarity.
15
(b)
(a) light dark
∆R1 / Rmid (%)
R (kΩ)
1.8
1.2 Vg < Vt
Vg > Vt
0.6
8
-30 V
-8 8
-20 V
-8 8
-10 V
-8 8
0V
-8 8
V g = +10 V
-8
-30
0 Vg (V)
30
0
6
12 18 24 time (minutes)
30
FIG. 3: (a) R − Vg curve for exfoliated single-layer graphene on MoS2 . Inset: Band bending inside MoS2 at two extreme gate voltages which results in different pathways for the optically generated EHPs. (b) Response of the device to illumination by white light. The red (solid) lines represent the moments in time when light is turned on. The green (dashed) lines correspond to light being turned off.
R (kΩ)
1.05
Rl
1.00 Rp
0.95 Ri
0
600
1200 t (s)
1800
FIG. 4: Response of the system to light. Note that there are three levels for the resistance here. The initial dark value (Ri ), a value under continuous illumination (Rl ) and an intermediate persistent state which is attained when light is turned off (Rp ).
16
SET
RESET
(a) pulse sequence
light pulse gate voltage pulse
1.69
Vg= -17.5 V
0.92
∆R 2
R (kΩ)
R (kΩ)
Rp Vg = 5 V Rd
1.64
0.87 0
200
400
600
t (s)
(b)
-20 V
1.59 1.57
-25 V
1.13
R (kΩ)
1.11 -30 V
0.83 0.81
-35 V
0.64 0.62
-40 V
0.52 0.50
10
20
30
40
50
t (minutes)
FIG. 5: (a) Toggling of the system between two states. R vs t plot is shown illustrating the ‘set’ and ‘reset’ operations achieved by light and gate pulses, respectively. For both curves, ILED = 5 mA. Note that the transient spikes in R following the gate pulses have been removed throughout the manuscript. The vertical red (dashed) and green (dotted) lines are used to indicate the light and gate pulses, respectively. (b) Evolution of the switching action as function of Vg .
17
20 µA 2 µA
2000µA 200 µA
ILED
ILED
3
0.53
0.50
(a)
4
12 t (minutes)
20
-30 Vg (V)
-20 0.8
Rl
0.7
∆V = 5 V
}
Rp
10 V
∆R 2 Rd
0
}
(c)
}
0.84
R (kΩ)
∆R 1
}
R (kΩ)
-40
(d)
0.88 0.86
1
(b)
Vg = -40 V
∆R 2 /Rd(%)
R (kΩ)
0.56
5 2 µA 20 µA 200 µA 2000µA
20 V
30 V
1000 2000 t (s)
3000
0
400 t (s)
0.6
800
FIG. 6: (a) Switching action for four different light intensities at a given Vg (b) Percentage change in resistance as a function of Vg for various illumination intensities. The values of current flowing through the illumination LED are indicated. (c) Effect of the gate pulse height on the dark state recovery. (b) Effect of the width of the light pulse on the switching effect.
18
8
26
-30 V
2
R - R (Ω)
R - R (Ω) d
(b) 8
0
R - R (Ω)
(a)
d
-35 V
2 8
t (s) -40 V
0.1
1
10
-30 V
14
-40 V
2
100
1
10
100
t (s)
t (s)
(c)
1E2 1E3 1E4
d
8
Graphene
MoS2
p+ Si
SiO2
DOS Ec EF
localized states x
Vg
t=0 trapped holes
electrons
t>>τ 0 (persistent regime)
(d) 1
ILED
∆Vg (V)
2000µA 200 µA 20 µA 2 µA
R
∆R
R
0.1
∆Vg
time
3
4 (Vg - VD )
1/2
5 (V
1/2
Vg
)
FIG. 7: (a) Nearly relaxation free nature of persistent states at three different gate voltages. ILED = 2 µA has been used for 0.1 s to ‘excite’ the system. (b) Transition from logarithmic decay to nearly relaxation free state after photoexcitation with LED current 200 µA for 0.1s. Solid lines are guide to eye. Inset shows the trace of ‘ON’ state over long time scale (∼ 12 hrs). (c) Schematic showing the evolution of electron-hole distribution in the system as a function of time. t = 0 corresponds to the moment after the light pulse. (d) ∆Vg vs.
19
√
Vg − VD . This plot is
calculated using the data shown in Fig. 5(b). The inset illustrates the conversion of ∆R to ∆Vg . Note: Fig. a, b, c, and partially d are adapted from ref. [33].