Fabrication and In Situ Transmission Electron Microscope

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Fabrication and In Situ Transmission Electron Microscope Characterization of Freestanding Graphene Nanoribbon Devices Qing Wang, Ryo Kitaura, Shoji Suzuki, Yuhei Miyauchi, Kanzunari Matsuda, Yuta Yamamoto, Shigeo Arai, and Hisanori Shinohara ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b06975 • Publication Date (Web): 05 Jan 2016 Downloaded from http://pubs.acs.org on January 11, 2016

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Fabrication and In Situ Transmission Electron Microscope Characterization of Freestanding Graphene Nanoribbon Devices Qing Wang†, Ryo Kitaura*,†, Shoji Suzuki†, Yuhei Miyauchi‡, Kazunari Matsuda‡, Yuta Yamamoto§, Shigeo Arai§ and Hisanori Shinohara*,† †

Department of Chemistry & Institute for Advanced Research, Nagoya University,

Nagoya 464-8602, Japan; ‡ Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan; § High Voltage Electron Microscope Laboratory, Ecotopia Science Institute, Nagoya University, Nagoya 464-8602, Japan

KEYWORDS: :

graphene

nanoribbon,

freestanding

structure,

in-situ

TEM

characterization, bandgap, transport measurement

ABSTRACT: Edge-dependent electronic properties of graphene nanoribbons (GNRs) have attracted intense interests. To fully understand the electronic properties of GNRs, the combination of precise structural characterization and electronic property measurement is essential. For this purpose, two experimental techniques using freestanding GNR devices have been developed, which leads to the simultaneous characterization of electronic properties and structures of GNRs. Freestanding graphene has been sculpted by a focused electron beam in transmission electron microscope (TEM) and then purified and narrowed by Joule heating down to several nanometers widths. Structure-dependent electronic properties are observed in TEM,

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and significant increase in sheet resistance and semiconducting behavior become more salient as the width of GNR decreases. The narrowest GNR width we obtained with the present method is about 1.6 nm with large transport gap of 400 meV.

The so-called graphene nanoribbon (GNR) is one of the most remarkable nanomaterials showing physical properties that are drastically dependent on size and the edge structure.1 Theoretical works on GNRs predict that GNRs possess width-dependent bandgap and structure-dependent salient edge states.2 The existence of sizable bandgap in narrow GNRs, which is not the case in zero-gap semiconductor graphene, has attracted a great deal of attention because of the promising application of GNRs in future nanoelectronic devices.3-8 The spin-polarized gapless edge state, which appears at the zigzag edge of GNRs, has also attracted significant attention in regards to spin-dependent transport and all-carbon magnet.9,10 It

is

not

straightforward,

however,

to

experimentally

investigate

the

structure-dependent properties of GNRs. Electronic transport measurements on GNRs prepared by lithographical patterning or hydrogen etching have been performed, and these works have successfully shown the presence of transport gaps roughly dependent on the width of GNRs.1,11 The observed gap is not an intrinsic bandgap but the so-called transport gap, where the observed transport gap is strongly influenced by roughness of edges and substrates underneath. In addition, characterization of precise atomic structure of GNRs sitting on substrates is extremely difficult, which refrains from understanding GNR’s properties based on their structures.12-14 Johnson’s group


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has already reported several works about in-situ TEM characterization of GNRs, providing the possibility of studying intrinsic structure-dependent properties of GNRs.15-17 To this end, development of an experimental technique to fully understand structure-dependent properties of GNRs has been strongly desired to explore possibility of GNRs in nanoelectronic and nanospintronic devices.

Figure 1. A schematic representation of the present experiment. A substrate-free GNR suspended between electrodes is mounted on the top of a home-developed TEM holder for in-situ structure and electronic properties characterization. The electrodes of GNR devices are connected to the electrode pads by the wire-bonding, and characterizations of electronic properties have been performed through the pads.

The purpose of this work is to develop an experimental technique for investigation of intrinsic structure-dependent properties of GNRs. For this purpose, the precise and


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simultaneous characterization of structural and electronic property measurements is essential. We use TEM for structural characterization together with substrate-free suspended GNRs to investigate their intrinsic properties. Figure 1 shows a schematic representation of the developed technique. A substrate-free GNR suspended between electrodes is mounted on the top of a home-developed TEM holder for in-situ structure and electronic properties characterization. Using this newly developed experimental technique, correlating atomic structure of GNRs and their transport properties is realized. The suspended GNRs have been prepared by shaping of suspended graphenes. We have prepared the suspended graphenes by the following two different methods. In the first method, large-area graphene prepared by chemical vapor deposition (CVD) have been used. The CVD growth of graphene have been performed at 1323 K using methane and copper foils as a carbon source and substrates, respectively.18 After removal of the copper foil, graphene was directly transferred onto a pre-fabricated substrate possessing electrodes and a penetrating hole at the middle of the substrate (Figure 2a). The suspended graphene was then narrowed by high-power laser etching before the shaping process in TEM. In the second method, graphene prepared through the mechanical exfoliation of Kish graphite has been used. The graphene prepared by the exfoliation method was transferred onto a Si substrate with an open slit covered by a SiO2 thin film (Figure 2d), where graphene was precisely located at the slit part using the deterministic transfer method.19,20


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Figure 2. A schematic representation of the two different preparation procedures. In the first method: (a) the pre-fabricated substrate possessing Pt electrodes and a penetrating hole at the middle of the substrate; (b) and (c) CVD-grown graphene on the substrate after transfer and high-power laser etching. In the second method: (d) the Si substrate with an open slit covered by a SiO2 thin film; (e) a graphene FET-device fabricated on the substrate; (d) SiO2 film underneath graphene is etched away by buffered HF solution to get freestanding graphene.

To fabricate a freestanding graphene, we first made electric contacts on the transferred graphene with Cr/Au, and then the SiO2 thin film underneath the graphene was etched away by buffered hydrogen fluoride acid. Freestanding graphene prepared by the above two methods was shaped to be suspended GNRs in TEM through sculpting by electron-beam irradiation and Joule-heating-induced vaporization of


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carbon.21-23 The electron-beam sculpting allows us to precisely control the width of GNRs, and Joule-heating-induced vaporization provides fast thinning while keeping surface of graphene clean. During the narrowing process, width-dependent electronic transport, I-V curves and differential conductance were measured together with corresponding TEM images. This leads to direct correlated measurements between structure and electronic properties.

Figure 3. (a) A TEM image of a CVD-grown graphene suspended between the electrodes after transfer onto a pre-fabricated substrate and (b) the corresponding diffraction pattern of graphene. (c) A TEM image after narrowing of the suspended graphene by the high-power laser irradiation. (d) I-V curve of the freestanding graphene devices in (c).

RESULTS AND DISCUSSION


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Figure 3a shows a typical TEM image of graphene suspended between the electrodes; this structure was prepared with CVD-grown monolayer graphene through transfer onto a pre-fabricated substrate. Dark contrasts at both sides in the TEM image correspond to pre-patterned Pt electrodes, and weak contrast arising from graphene is seen between the electrodes (dark spots on the graphene arise from residues of Cu used in CVD growth). The electron diffraction pattern of suspended graphene shows sharp spots with six-fold symmetry (Figure 3b), which clearly demonstrates the successful transfer of monolayer graphene. Figure 3c shows a TEM image after narrowing of the suspended graphene by high-power laser irradiation. As seen in Figure 3c, width of graphene is narrowed down to 10 µm, and edges of the narrowed graphene are rolled up, which gives linear contrasts at edge regions. The rolling-up of edges is probably caused by vibration of the edges that originates from laser-induced high temperature. Figure 3d shows an I-V curve of the narrowed graphene. The observed linear relation between I and V provides resistivity of about 9.0 × 10-7 Ω·m. The obtained resistivity is comparable to reported values (about 2.0 × 10-7 Ω·m) measured by two-terminal devices.24 Figures 4a and 4b show TEM images of a suspended graphene before and after shaping, respectively. The suspended graphene was prepared using the mechanical exfoliation method. The width of the graphene shown in Figure 3a is 430 nm, and the dark contrasts, which locate at the top and the bottom of the image, correspond to the Cr/Au electrodes deposited on the graphene. Before preparation of this suspended structure, monolayer structure of the graphene was confirmed by Raman spectroscopy


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and transport measurements (see details in Figure S1). After electron-beam sculpting, a narrow GNR with a width of 5 nm was obtained (Figure 4b). In the sculpting process, acceleration voltages of 120 and 200 kV were used to prompt the sculpting of graphene. During the sculpting, the graphene heating was maintained by Joule heating to heal atomic defects and to prevent the electron beam induced amorphous carbon contamination.25,26

Figure 4. (a) and (b) TEM images of a suspended graphene before and after shaping with widths of 430 nm and 19 nm, respectively. (c) A close-up image of the graphene after narrowing (the inset shows corresponding FFT image). (d) Resistance of the suspended graphene as a function of its width (varying from 500 to 5 nm). The dashed line is a fitting line based on the equation of R = s / w + 2Rc, where s, w, and Rc corresponds to a constant, width and contact resistance, respectively.


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As we can see in Figure 4b, the surface of the graphene is clean with some folded region, in particular, around the edges. The width of the folded or rolled part is small compared to total width of GNRs (less than 5 %), and the change in electronic structure caused by formation of folded or rolled edges has been proved not to be significant.27 The closed-up image shown in Figure 4c clearly demonstrates the appearance of lattice fringe without amorphous contaminations, indicating that Joule heating have provided the clean surface. The Fourier-transformed image of Figure 4c shows spots with 6-fold symmetry, and d-value of the spots corresponds to (110) plane of graphene (0.21 nm). In the narrowing-down process, we measured resistances as a function of width varying from 500 to 5 nm (Figure 4d), and the data is well-fitted by the function of R = s / w + 2Rc, where s, w, and Rc corresponds to a constant, width and contact resistance, respectively; the fitting gives 3.77 × 103 kΩ⋅nm and 11.6 kΩ for s and Rc, respectively. A theoretical calculation of armchair GNR bandgap using a first principle many-body Green’s function approach under GW approximation has shown that the bandgap, Eg, is described by the following formula, Eg = a/(w+2.4+δ), where a and δ are parameters ranges from 14.6 ~ 44.4 eVÅ and 1.8 ~ 2.9 Å, respectively, and w corresponds to the width of GNR.28 The fact that the function (R = s / w + 2Rc) reproduces the data indicates that resistivity of the GNR is almost proportional to 1/w, which indicates ultra-narrow GNRs is required to see bandgap formation explicitly through transport experiments.


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Figure 5. (a) TEM images of the graphene having an ultra-narrow region, about 1.6 nm wide. (b) The current and differential conductance as a function of bias voltage.

The

combination

of

electron-beam

sculpting

and

Joule-heating-induced

vaporization of carbon atoms has finally led to the successful fabrication of an ultra-narrow GNR with a width of 1.6 nm. Electron-beam sculpting alone cannot yield such an ultra-narrow structure because spatial drift of a sample during the sculpting disturbs precise positioning of the sample. Figure 5a shows a TEM image of the graphene having an ultra-narrow region. The narrowest part of the graphene is much narrower than the remaining part, which can be considered as electrodes connected smoothly to the narrowest part. In this case, the electronic transport property is predominantly determined by the 1.6 nm wide structure. Figure 5b shows the current


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and differential conductance as a function of bias voltage, in which the differential conductance of the GNR is in close proximity to zero when the bias voltage approaches zero. It should be noted that the value of dI/dV at zero bias voltage is not zero presumably due to the back gate leak current from substrate and tunneling current through the GNR. Observation of nonlinear transport characteristics is one of the important approaches to probe the transport gap of GNRs.29-31 In the transport gap regime, transport through the GNR at finite bias voltages shows a strong nonlinear I-V characteristic when gate voltage is near the charge neutrality point of the GNR, where the transport gap can be estimated as half of nonlinear gap (∆V).29 The ∆V is estimated to be 800 meV based on the I-V curve shown in Figure 4b, indicating that the 1.6 nm wide structure has a transport gap of about 400 meV. The observed transport gap of 400 meV is the largest among GNRs currently observed through transport measurements. A GW calculation28 has shown that 1.6 nm armchair GNR and zig-zag GNR have bandgaps of 740 ~ 2100 meV and 1200 meV, respectively, which are larger than the observed value. In the present experiment, the length of the ultra-thin GNR part is short, and the large graphene part connected to the GNR may affect electronic structure of the channel part. We think that this may affect the transport gap measurement. In addition, the structure of the ultra-narrow GNR is not straight but slanted, and this can also affect the electronic structure of GNRs. To investigate the effect of a slanted edge on the bandgap of a GNR, we have performed molecular orbital calculations (the semiempirical method based on PM632) of the three different


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graphene nanostructures (structural models and calculation results are given in supporting information). The results of the molecular orbital calculations indicate that the slanted GNR probably possess bandgap that is smaller than that of GNR with width of 1.6 nm, which is consistent to the observed smaller bandgap. This suggests that shape of GNR, in particular in the case of a short GNR, can have an important implication on electronic structure and transport properties of GNRs.

Based on these,

we have concluded that the intrinsic bandgap of the GNR have successfully been observed through transport experiments, which clearly demonstrates that the newly developed technique is a powerful tool to investigate electronic properties of various ultra-small 2D materials. CONCLUSIONS In conclusion, we reported two kinds of new experimental techniques to fabricate freestanding graphene devices. Utilization of electron beam sculpting and Joule heating in TEM, ~ 10 nm wide GNR can be obtained. In the narrowing-down process, the conductance of GNRs behaves nonlinearly as a function of width, indicating the opening of bandgap I-V and conductance differential characteristic are performed with known GNR structures. The thinnest GNR structure we obtained is about 1.6 nm wide with an estimated transport gap about 400 meV. The observed transport gap of 400 meV is the largest among GNRs so far reported through transport measurements, and this method can be used to study the intrinsic electronic properties of GNRs. The present experimental technique should be used to correlate the electronic properties with known structure in two dimensional materials such as transition metal


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dichalcogenides. METHODS Preparation of graphene suspended structures. Graphene used in the first method was prepared in non-pressurized CVD at 1323 K using methane as a carbon source and a copper foil as a substrate.18 Prior to CVD growth, a copper foil (nilaco, 99.9 %) was annealed at 1323 K under 500 sccm flow of 100 % hydrogen for 50 minutes. The growth of graphene was carried out under 250 sccm flow of Ar/H2/CH4 (Ar: 97 %, H2: 3 %, CH4: 0.0002 %) mixture gas for typically 2 hours. After the CVD growth, the copper substrate was dissolved in FeNO3 solution and washed in purified water for several hours. Graphene was then directly transferred onto a pre-fabricated substrate shown in Figure 2a. In the second method, graphene prepared through the mechanical exfoliation of Kish graphite has been used. First, graphene flakes were peeled off from Kish graphite using Nitto adhesive tape (31B) and then pasted onto a PDMS film. Monolayer or few-layer graphenes were located on PDMS by an optical microscope, and the layer-numbers of each graphene were verified by Raman spectroscopy. The graphene with known layer-number was transferred onto a Si substrate with an open slit covered by a SiO2 thin film (Figure 2d), where graphene was precisely located at the slit part using the deterministic transfer method.19,20 Device Fabrication. To fabricate graphene FET devices in the second method, we first made electric contacts on a transferred graphene with Cr/Au using electron beam lithography (ELS-3700NC, ELIONX) and thermal evaporator. The SiO2 thin film


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underneath the graphene was then etched away by buffered hydrogen fluoride acid, which gives a suspended graphene FET device. During the etching process, we covered graphene by a thin PMMA layer to avoid unwanted breakdown of the suspended structure. We removed the thin PMMA and cleaned the suspended graphene by a mild H2 plasma treatment. TEM observations. TEM observations were carried out using a high-resolution field-emission gun TEM (JEM-2100F and JEM-ARM200F, JEOL) operated at 120 or 200 kV at room temperature and under a pressure of 10-6 Pa. TEM images were recorded with a charge coupled device with an exposure time of typically 1 second. Electrical measurement. Measurements of transfer and output characteristics of exfoliated graphenes were performed in a vacuum probe station (ST-500-1-4TX, JANIS) and semiconductor parameter analyzer (4200-SCS, Keithley). We used a nanovoltmeter (2182A, Keithley) and an AC/DC current source (6221, Keithley) for the in-situ measurements of I-V curves and differential conductance. ASSOCIATED CONTENT

Supporting Information

Raman spectrum and transport measurements of exfoliated graphene device; Structural models and calculation results of the presented GNR.

This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors


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*E-mail [email protected] & [email protected]

ACKNOWLEDGEMENTS

This work was supported by Grant-in-aid for Young Scientists A (No. 25708002), Scientiﬁc Research on Innovative Areas (No. 25107002 and No. 25107003) and Scientiﬁc Research S (No. 22225001) from MEXT, Japan, and the Global COE Program in Chemistry, Nagoya University. We thank Prof. K. Nagashio for his kind advices on the dry transfer of graphene.

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ACS Nano

(29) Han, M. Y.; Brant, J. C.; Kim, P. Electron Transport in Disordered Graphene Nanoribbons. Phys. Rev. Lett. 2010, 104, 056801. (30) Gallagher, P.; Todd, K.; Goldhaber-Gordon, D. Disorder-Induced Gap Behavior in Graphene Nanoribbons. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 115409. (31) Lin, M.-W.; Ling, C.; Agapito, L. A.; Kioussis, N.; Zhang, Y.; Cheng, M. M.-C.; Wang, W. L.; Kaxiras, E.; Zhou, Z. Approaching the Intrinsic Band Gap in Suspended High-Mobility Graphene Nanoribbons. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 1254111. (32) Stewart, J. J. P. Optimization of Parameters for Semiempirical Methods V: Modification of NDDO Approximations and Application to 70 Elements. J. Mol. Model. 2007, 13, 1173-1213.


ACS Nano

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Figure for table of contents 254x158mm (96 x 96 DPI)


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Fabrication and In Situ Transmission Electron Microscope

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