Temperature dependent carrier mobility in graphene: Effect of Pd nanoparticle

Transcription

1 Temperature dependent carrier mobility in graphene: Effect of Pd nanoparticle functionalization and hydrogenation Bochen Zhong 1*, Md Ahsan Uddin 2,3*, Amol Singh 2, Richard Webb 1 and Goutam Koley 2,3 1 Department of Physics and Astronomy, University of South Carolina, Columbia, SC 29208, USA 2 Department of Electrical Engineering, University of South Carolina, Columbia, SC 29208, USA 3 Department of Electrical and Computer Engineering, Clemson University, Clemson, SC 29634, USA uddin2@ .sc.edu; gkoley@clemosn.edu *These authors contributed equally to this work 1

2 Abstract: The two dimensional (2D) nature of graphene, with charge carriers confined within one atomic layer thickness, causes its electrical, optical and sensing properties to be strongly influenced by the surrounding media and functionalization layers. In this study, the effect of catalytically active Pd nanoparticle (NP) functionalization and subsequent hydrogenation on the hall mobility and carrier density of chemical vapor deposition (CVD) synthesized graphene has been investigated as a function of temperature. Prior to functionalization, the mobility decreased monotonically as the temperature was reduced from 298 to 10 K, indicating coulomb scattering as the dominant scattering mechanism as expected for bilayer graphene. Similar decreasing trend with temperature was also observed after 2 nm Pd deposition, however, hydrogenation of the Pd NP led to significant enhancement in mobility from ~2250 to 3840 cm 2 /Vs at room temperature, which further monotonically increased to 5280 cm 2 /Vs at 10 K. We attribute this contrasting trend in temperature dependent mobility to a switch in the dominant scattering mechanism from coulomb to surface optical (SO) phonon scattering due to higher dielectric constant and polar nature of PdH x formed upon hydrogenation of the Pd NPs. 2

3 Since its first demonstration [1], graphene has been extensively studied over the past decade due to its outstanding electrical [2,3], optical [4], mechanical [5] and other material properties. Charge carriers in graphene are confined in a layer of ~0.34 nm thickness, making surface functionalization an attractive approach for enhancing the performance of existing devices and enabling new applications. Metallic nano-particle (NP) functionalization of graphene is an important technique that not only changes its doping and carrier density but also facilitates adsorption and consequent detection of analytes that is not possible with pristine graphene. Thus, metallic NP functionalization can impart chemical sensitivity [6, 7] as well as help in tuning device characteristics [8], and even enhance their optical properties [9]. As an example, while non-functionalized graphene-based gas sensors are sensitive to an extensive set of polar molecules such as, NO 2 and NH 3 [10-13], non-polar molecules, such as H 2, can only be detected through appropriate NP functionalization [6, 7]. Although, extensive research reports have been published on gaseous detection based on amperometric sensing mechanism, most of those are focused on the change in carrier density due to charge interaction between graphene and the adsorbates. Detailed studies focusing on change in carrier mobility and scattering mechanism due to surface functionalization and analyte adsorption are notably absent, although they can provide very important insight into the ultimate sensitivity limits of sensors, as well as the performance enhancement of electronic devices such as field effect transistors. In this work, temperature dependent Hall mobility of CVD synthesized graphene has been investigated before and after Pd NP functionalization and subsequent hydrogenation. The mobility was observed to decrease as the temperature decreased from 298 K to 10 K, a trend that was still observed after 2 nm Pd NP deposition, although the magnitude of mobility decreased. In contrast, after hydrogenation of the Pd NPs a strongly increasing trend of mobility as a function 3

4 of temperature was observed. The observed mobility trend is explained by large dielectric constant and polar nature of PdH x formed upon hydrogenation of the Pd NPs, incorporating variation in the dominant scattering mechanism. Moreover, n-type doping of graphene by Pd NPs as obtained from Hall measurement is explained considering the graphene-pd NP interaction distance. High quality graphene was synthesized on Cu foil through chemical vapor deposition (CVD) process in a quartz tube furnace at 1035 and 10 Torr pressure with CH 4 (flow rate of 40 sccm) and H 2 (flow rate of 50 sccm), as described in detail elsewhere [14,15]. To minimize the fabrication induced contamination of graphene, metal contact pads (for graphene) were deposited on SiO 2 before the graphene transfer. The contact pads were patterned on SiO 2 by photolithography followed by metal deposition [Ti (20nm)/Au (80nm)] by electron beam evaporation at a base pressure of 10-6 Torr. Next, graphene was transferred following wellestablished PMMA based wet transfer procedure [16,17]. Finally, the transferred graphene was patterned into Hall bars through a second photolithography step followed by oxygen plasma etching, where the graphene channel dimension is nominally 600 µm 100 µm. After initial material and electrical characterizations (including temperature dependent Hall measurements), 2 nm Pd was deposited on the graphene channel region using a shadow mask by electron beam evaporation. Hydrogenation of the Pd NPs was carried out by exposing the functionalized graphene surface to 1000 ppm H 2 (diluted using N 2 ) for 60 minutes. Figure 1 exhibits a high resolution SEM image of 2 nm Pd NP decorated graphene. Due to slow growth rate (~0.2 Å/s) and the short deposition time (~100 seconds for 2 nm), the Pd grains did not coalesce to form a complete film, so the graphene film acts as the current carrying channel, albeit with modified electrical properties. The inset in Fig. 1 shows an atomic force 4

5 microscope (AFM) surface morphology image which also underlines the discontinuous yet fairly uniform coverage of NPs on graphene (which is folded at places) with a surface RMS roughness of 3.8 nm. Figure 2 shows the Raman spectra for as-grown, Pd NP functionalized and hydrogen exposed Pd-NP functionalized graphene. 2D/G ratio of 1.65 and 2D peak full width half maximum (FWSM) of ~34 cm -1 indicates the graphene film is bilayer. Initial D/G peak intensity ratio is around 0.1, which does not change significantly after Pd NP deposition, indicating no significant film degradation or structural change happened after Pd NP functionalization. Following 1000 ppm H 2 exposure, the G peak was found to shift from to cm -1 while the width increased from 13.4 to cm -1 and 2D/G intensity ratio increased from 1.65 to Red shift in Raman G peak position with molecular exposure has been related to n-type doping [18], while the increase in G peak width and 2D/G peak ratio indicates reduction in impurity induced carrier density [19,20] which is consistent with reduction in p-type doping of graphene by PdH x and discussed in detail later. For temperature dependent Hall measurements, the samples were investigated in Quantum Design Physical Property Measurement System (PPMS) from room temperature to 10 K. Typical four-terminal method was used for the measurements. A constant dc current was applied through the device and the voltages V xx and V xy (also known as the Hall voltage V H ) were measured across the terminals with the magnetic field B varying up to 8 T applied perpendicular to the graphene film. The carrier concentration p (our graphene is p-type) is derived from the Hall voltage V H : p = IB/eV H, and the Hall mobility μ is given by the relation: μ = σ/ep, where σ is the four-probe conductivity measured along the current direction [21]. Magneto-transport studies on graphene were carried out using pristine graphene and then after 2 nm Pd NP functionalization over the temperature range from 298 to 10 K. The Hall mobility and carrier density variation for 5

6 a representative sample under these two conditions are shown in Fig 3(a). For pristine graphene, the mobility can be seen to decrease monotonically from 2405 cm 2 /Vs at room temperature to 2262 cm 2 /Vs at 10 K. This type of decreasing trend has been reported in literature as the transport characteristics for bilayer graphene [22], which is consistent with the Raman spectra of non-functionalized graphene discussed earlier. To ensure the magnetoresistance effects are not significant we extracted the magnetoresistance factor from the Hall measurement data, which turned out to be only ~1 % over the temperature range of measurement. Therefore, we neglected any magnetoresistance effect in our calculations. In case of bilayer graphene, the additional graphene layer screens the substrate surface polar phonon scattering originating from SiO 2 optical phonons at the substrate/graphene interface, and Coulomb scattering is the dominant scattering mechanism [23, 24]. Coulomb scattering time averaged over energy for parabolic band structure of bilayer graphene results in mobility increase with temperature [22, 25]. The variation in carrier concentration of the pristine graphene can also be seen from Fig. 3(a), which is cm 2 at 298 K, and remains almost constant over the entire temperature range down to 10 K. After 2 nm Pd NP deposition on graphene, the mobility still exhibited a decreasing trend with temperature [Fig. 3(a)] with the mobility magnitude reduced slightly at a given temperature. This reduction can be attributed to additional scattering sources introduced by the functionalization layer. Considering the higher work function of Pd compared to graphene ( ev [26, 27]) compared to ~4.5 ev [28]), p-type doping is normally expected in Pdfunctionalized graphene [6, 29]. Interestingly however, the carrier concentration of Pdfunctionalized graphene is found to be cm 2 at room temperature, which is less than cm 2 obtained before functionalization, indicating an n-type doping effect [Fig. 3(a)]. In 6

7 order to explain this unexpected behavior of Pd NPs we consider the numerical and analytical models developed by Giovannetti et al. [28] regarding interaction between graphene and various metals (such as Al, Ag, Cu, Au, and Pt), which form weak bonding with graphene while preserving their individual electronic structure. Although it was proposed that graphene electronic structure can be significantly altered by Pd (111), its thickness is only 2 nm in our study, which does not form a continuous layer as seen from the SEM image, and did not result in any noticeable change in the Raman spectra [Fig. 2(a)]. Considering these observations, we have employed the weak bonding model to explain the findings for Pd NP decorated graphene in the present work. The charge redistribution at the graphene-metal interface not only depends on the electron transfer between the metal and the graphene due to difference in their work functions, but is also affected by the graphene-metal chemical interaction which is a strong function of the distance between graphene and metal. When the graphene-metal distance is greater than 4.2 Å, the potential due to graphene-metal chemical interaction is negligible, and charge transfer is governed primarily by the difference in work function. However, for smaller distances, the chemical interaction induced potential becomes sizable which increases the effective graphene work function (W G ) significantly, as for example, at the equilibrium distance of 3.3 A, theoretically obtained W G of 5.4 ev much higher than its freestanding value of ~4.5 ev [28]. Defining metal work function as W M and V(d) as the potential change generated by the metalgraphene chemical interaction, Fermi level shift in graphene, E F (d) as a function of the graphene-metal surface distance (d) can be given by the following equation, E F (d) = W M V(d) W G (1) Fermi level shift, E F (d) as a function of the graphene-metal surface distance (d) for different weakly interacting metal is shown in Fig. 3(b) [28] where only for Pt which has highest work 7

8 function, E F is positive for all d and resultant doping in graphene is always p-type. Since the work function of Pd is smaller than Pt but larger than Au, the trend for Pd NP doping (based on weak interaction as discussed above) should be in between those of Pt and Au as shown by the broken red line in Fig. 3(b). For very small interaction distance d between Pd NP and graphene, E F would be negative and resultant doping is n-type, reducing the p-doing of CVD graphene, and opposite trend should be observed for large separation distance. Although the distance d is difficult to determine, similar measurements conducted on other samples indicate Pd NP induced p-type doping of graphene is also possible (See supplemental material at [URL will be inserted by AIP] for p-type doping of graphene by Pd NP as shown in Fig. S1 [30]). To investigate the effect of variation in work function of the Pd NP dopants by hydrogenation on graphene electronic properties, we exposed the Pd functionalized graphene to 1000 ppm H 2, and performed temperature dependent (room temperature to 10 K) Hall measurements. The variations in mobility and carrier density are shown in Fig. 4. We find that exposure to H 2 reduces the hole density in graphene from to cm 2 at room temperature. Since H 2 molecule interacts with Pd and dissociates into atomic hydrogen forming PdH x [31], which has a lower work function than graphene [32], we can expect more electrons to transfer to graphene reducing its hole carrier density, in agreement with the experimental observations. As the temperature decreases, the carrier concentration decreases gently but monotonically to cm 2 at 10 K following similar trends as pristine and Pd NP decorated graphene [Fig. 3(a)]. However, more interesting effects of H 2 adsorption was observed regarding carrier mobility. We found the mobility to increase significantly from 2279 to 3840 cm 2 /Vs, following exposure to hydrogen, at room temperature and the increasing trend continued to 10 K, in sharp contrast with the decreasing trends observed for pristine and Pd decorated graphene 8

9 [Fig. 3(a)]. The measurements were repeated over several samples and over a duration of several months and similar trend of mobility as a function of temperature was obtained (See supplemental material at [URL will be inserted by AIP] for PdH x induced mobility enhancement of other sample and repeatability as shown in Fig. S1 and S2, respectively [30]). This trend of increasing mobility with decrease in temperature is generally related to phonon scattering, and similar trend has been reported earlier for non-functionalized monolayer graphene on SiO 2 substrate where substrate surface polar phonon scattering was determined to be the predominant scattering mechanism [22]. This would imply that the dominant transport mechanism for H 2 exposed Pd-functionalized bilayer graphene switched form Coulomb scattering (see earlier discussion) to surface optical phonon scattering. Due to its high dielectric constant, PdH x can help in screening Coulombic scattering causing the mobility to increase, which has been predicted theoretically [33] and observed experimentally [34, 35]. By coating 2D semiconductor nanostructures with high-κ dielectrics, the Coulomb scattering inside the semiconductor can be dramatically damped so that the mobility improves as much as an order of magnitude [33]. Such mobility enhancements in graphene has been observed by coating with conventional high-κ dielectrics such as HfO 2 and Al 2 O 3 [34, 35]. Since the dielectric constant of PdH x is larger than air [36], it makes the sample a sandwich structure of dielectric/graphene/dielectric, enhancing the carrier mobility by almost a factor of ~1.7 at room temperature. Additionally, although not a part of this study, graphene encapsulation by material of high dielectric constant is expected to result in 1/f noise reduction as reported in literature [37, 38]. In conclusion, we have investigated the temperature dependent transport properties of Pd nanoparticle functionalized and hydrogenated CVD grown bilayer graphene through Hall measurement. In pristine graphene sample, the Hall mobility decreases with temperature from 9

10 298 to 10 K, indicating that the mobility is dominated by Coulomb scattering. After 2 nm Pd deposition, the mobility reduces due to additional source of scattering induced by Pd functionalization layer although the decreasing trend remains the same. In contrast, upon hydrogen exposure, strong enhancement in mobility has been observed, which increased monotonically with decrease in temperature. The improvement in transport property can be attributed to a reduction in Coulombic scattering due to encapsulation by hydrogenation of Pd, resulting in thin layer of PdH x with high dielectric constant, in agreement with theoretical predictions and experimental demonstrations of carrier mobility enhancement in semiconductor nanostructures through high-κ dielectric engineering. Financial supports for this work from National Science Foundation (Grants Nos. ECCS , ECCS , and ECCS ) are thankfully acknowledged. 10

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14 Figure Captions: Fig. 1. SEM image of 2 nm Pd NP functionalized graphene showing graphene folds, which is also seen in AFM image (inset) of Pd NP functionalized graphene along with nanoparticles. Fig. 2. Raman spectra of bilayer graphene on SiO 2 /Si substrate before and after 2 nm Pd deposition, and after H 2 exposure. Fig. 3. (a) Hall mobility and carrier concentration plotted as a function of temperature for pristine graphene and Pd decorated graphene. (b) Fermi level shifts E F (d) for various metals plotted as a function of the graphene-metal surface distance d (reproduced with permission from Phys. Rev. Lett. 101, (2008); copyright 2008 American Physical Society [25]). The dots represent Fermi level shifts calculated numerically from first-principles based on density functional theory, while the solid lines represents the variations obtained from analytical Model. The broken red line indicates the approximate trend for Pd NP, with work function in between Pt and Au, which is used to explain the Pd NP induced doping of bilayer graphene in this study. Fig. 4. Hall mobility and carrier concentration plotted as a function of temperature for PdH x covered graphene showing an increasing trend in Hall mobility and decreasing trend in carrier density with decrease in temperature, the former is in sharp contrast with pristine and Pd functionalized graphene (shown for comparison). 14

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