Seamless Staircase Electrical Contact to Semiconducting Graphene

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Seamless staircase electrical contact to semiconducting graphene nanoribbon Chuanxu Ma, Liangbo Liang, Zhongcan Xiao, Alexander A Puretzky, Kunlun Hong, Wenchang Lu, Vincent Meunier, Jerzy Bernholc, and An-Ping Li Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02938 • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 7, 2017

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Seamless staircase electrical contact to semiconducting graphene nanoribbon Chuanxu Ma1, Liangbo Liang1*, Zhongcan Xiao2, Alexander A. Puretzky1, Kunlun Hong1, Wenchang Lu1,2, Vincent Meunier3, J. Bernholc1,2, An-Ping Li1* 1

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 2

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Department of Physics, North Carolina State University, Raleigh, NC 27695, USA

Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180, USA *Email: [email protected]; [email protected]

ABSTRACT: Electrical contact to low-dimensional (low-D) materials is a key to their electronic applications. Traditional metal contacts to low-D semiconductors typically create gap states that can pin the Fermi level (EF). However, low-D metals possessing limited density of states at EF can enable gate-tunable work functions and contact barriers. Moreover, a seamless contact with native bonds at the interface, without localized interfacial states, can serve as an optimal electrode. To realize such a seamless contact one needs to develop atomically precise heterojunctions from the atom up. Here, we demonstrate an all-carbon staircase contact to ultra-narrow armchair graphene nanoribbons (aGNRs). The coherent heterostructures of width-variable aGNRs, consisting of 7, 14, 21 and up to 56 carbon atoms across the width, are synthesized by a surface-assisted self-assembly process with a single molecular precursor. The aGNRs exhibit characteristic vibrational modes in Raman spectroscopy. A combined scanning tunneling microscopy and density functional theory study reveals the native covalent-bond nature and quasi-metallic contact characteristics of the interfaces. Our electronic 1

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measurements of such seamless GNR staircase constitute a promising first step towards making low resistance contacts. Keywords: Electrical contact, graphene nanoribbon, heterostructure, staircase, scanning tunneling microscopy, vibrational modes

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A key issue in fabricating electronic devices is making a good electrical contact to the semiconductor gate material.1-3 Inappropriate contacts create interfacial states that can pin the Fermi level (EF) and form a big Schottky barrier.4 The conventional approach, reducing the width of the depletion region through local doping, is not available for low-dimensional materials.5 For two-dimensional (2D) transition metal dichalcogenides (TMDs), a route to a high-performance contact has recently been proposed by using a phase transition that converts a hexagonally packed semiconductor (2H) phase into a distorted octahedrally packed metallic (1T’) phase.6, 7 However, a similar approach is not available for 1D materials. Moreover, while a metallic contact has been demonstrated by aligning the metal work function with the energy band of the 2D semiconductor, a structural mismatch still exists at the TMD interfaces, which could create a barrier for charge transfer.8 Conceptually, an ideal contact would be a metal-semiconductor interface formed with native covalent bonds without introduction of any structural or electronic boundaries.2,

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Realization of such a seamless contact requires atomically precise development of the heterostructures from well-defined atomic or molecular precursors. Here based on the ultra-narrow graphene nanoribbons (GNRs), a wide bandgap 1D semiconductor that has been studied extensively,12-22 we report on a successful approach for making seamless contacts in 1D materials through the formation of an atomic staircase heterostructure. The coherent staircase is made of armchair graphene nanoribbons (aGNRs) with widths varying from 7, 14, 21 and up to 56 carbon atoms, and lengths from sub-nanometer to 100 nm. The graphitic heterostructures are synthesized by a surface-assisted self-assembly process with a single molecular precursor. While the 7-atom wide aGNR (7-aGNR) is a large-gap semiconductor, the conjugated wide GNRs are either quasi-metallic or small-gap semiconductors, similarly to the 2D metals. Our 3


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study, which combines scanning tunneling microscopy (STM) and Raman measurements with density functional theory (DFT) calculations, reveals that the heterointerface consists of native sp2 carbon bonds without localized interfacial states. Such a seamless heterostructure thus offers an optimal electrical contact to the wide-gap 1D semiconductors. The 10,10’-dibromo-9,9’-bianthryl (DBBA) molecules were adopted as the precursor to grow the 7-aGNRs on an Au(111) substrate in a two-step polymerization (at 470 K) and cyclodehydrogenation (at 670 K) process, as reported previously12, 23 (Methods). Figure 1a illustrates the synthesis procedure of the GNR heterojunctions (HJs). After the formation of 7-aGNRs, the sample was further annealed at 770 K and the 14 or 21-aGNRs were formed when two or three 7-aGNRs conjugated side-by-side via inter-ribbon cyclodehydrogenation at the edge sites13 (see Figure S1 in Supporting Information). The ratio of the fused ribbons to the total number of 7-aGNRs depends on the coverage (Figure S2). For example, at coverages of about 0.2 and 0.6 monolayers, the fusion ratios are about 18% and 45%, respectively. Among the total number of ribbons, the ratios of the newly formed 14-aGNRs and 21-aGNRs at 0.2 monolayer coverage are about 10% and 1.5%, respectively, which change to about 16% and 6% at 0.6 monolayer coverage. Wider graphitic flakes with widths N (in carbon atoms) of 4 to 8 times of the 7-aGNR were formed after annealing at 870 K (Figure S3). These GNRs possess width-dependent electronic properties according to first-principles calculations.24 For example, the 7 and 21-aGNRs belong to the groups of N = 3n + 1 (n is an integer) and N = 3n, respectively, which are predicted to have bandgaps that scale inversely with the ribbon width. In contrast, the 14-aGNR, belonging to the group of N = 3n + 2, is predicted to be metallic or quasi-metallic25 with a small bandgap of about 0.2 eV (ref 24). Thus, a variety of HJs made of semiconducting or quasi-metallic GNRs can be realized by fusing them together. 4


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Figure 1. Bottom-up synthesis of graphene nanoribbon (GNR) heterojunctions (HJs). (a) Schematic procedure of GNR HJ synthesis with 10,10’-dibromo-9,9’-bianthryl (DBBA) molecules after three-step annealing at 470 K (polymerization), 670 K (graphitization), and 770 K (conjugation) respectively. After annealing at 870 K, graphene-like wide GNRs with widths N up to 56 carbon atoms can form (Figure S3). (b–d) Scanning tunneling microscopy (STM) images of the 7, 14 and 21-aGNRs respectively, superposed with the atomic structures (sample voltage Vs = −0.5 V, tunneling current It = 200 pA). (e) STM image of a 7–14 aGNR HJ and, (f) a 7–14–21 aGNR HJ (Vs = −0.6 V, It = 100 pA). All scale bars, 1 nm.

The STM images of the 7, 14 and 21-aGNRs are shown in Figure 1a-d with apparent widths of approximately 1.5 nm, 2.2 nm and 3.1 nm, respectively. The lengths of these GNRs can vary between sub-nm to 100 nm (refs 12, 14) (Figure S1). STM images in Figure 1e,f show the interface structures of a 7–14 and a 7–14–21 aGNR HJ, respectively, where GNRs with different widths are connected along the armchair direction. As further confirmed by the simulated STM image (Figure S1), the bonds at these HJ interfaces are native sp2 carbon–carbon bonds without structural defects, 5


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unlike the interfaces formed by two perpendicular aGNRs (Figure S4) where the pentagon and heptagon ring pair is observed at the T-junction.21

Figure 2. Width dependent Raman spectra of aGNRs. (a) Raman spectra of GNRs formed after annealing at 670 K (black), 770 K (red) and 870 K (blue), respectively. The peaks at 396, 262, and 955 cm-1 correspond to the characteristic breathing-like mode (BLM), the shear-like mode (SLM), and another kind of BLM (BLM3) of the 7-aGNR, respectively. The peak at 199 cm-1, which is only present after 770 K annealing and disappears after 870K annealing, corresponds to the BLM of the 14-aGNR. (b) Sketches of the observed vibration modes marked with colored short lines in (a). (c) Width-dependent wavenumbers of the BLM and SLM, calculated using density functional theory (DFT) and compared with experimental results on the 7-aGNR,12 9-aGNR,26 and 14-aGNR.

The variable width of GNRs formed at different annealing temperatures is confirmed by Raman 6


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spectroscopy. Figure 2a shows the Raman spectra (see Methods for details) acquired from densely packed layers of 7-aGNRs after annealing at temperatures of 670 K, 770 K and 870 K, respectively (full range Raman spectra are provided in Figure S5). After 870 K annealing, the Raman spectrum (blue) resembles that of the 2D graphene,27 indicating the formation of wide sp2 graphitic sheets (Figure S3). The width-sensitive Raman modes appear below 1000 cm-1, as illustrated in Figure 2b. The main peak at 396 cm-1, known as the characteristic breathing-like mode (BLM) of the 7-aGNR,12 is more pronounced after 670 K (black curve) and 770 K (red curve) annealing, with a weak double overtone at 792 cm-1 shown in Figure 2a. Another type of BLM of the 7-aGNR, BLM3,28 is observed at 955 cm-1. Interestingly, a peak, never reported before, appears at 262 cm-1 in both samples. Based on our DFT calculations, it corresponds to a shear-like mode (SLM) of the 7-aGNR (Figure 2b). After annealing at 770 K to form the 14 and 21-aGNRs, a low-intensity peak emerges at 199 cm-1, corresponding to the BLM of the 14-aGNR. No characteristic peaks of 21-aGNRs are observed possibly due to their low concentrations. Figure 2c compares the calculated peak positions with measured values for the BLM and SLM modes with varying aGNR width26, 28, 29 (more details in Figure S6 and Table S1). Good agreement can be seen. These Raman modes offer characteristic signatures for quick verifications of the quality and width of GNRs. Observation of the characteristic vibration modes of GNRs also confirms their stability in ambient conditions as the Raman measurements were carried out in air.

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Figure 3. Band alignments in the 7–14 aGNR heterojunction. (a) STM image of a 7–14 aGNR HJ acquired at Vs = –2.0 V, It = 80 pA for the differential tunneling conductance dI/dV measurements. Scale bar, 2 nm. (b) The log(dI/dV) curves acquired across the HJ interface along the solid red lines marked in (a) (equally vertically shifted for clarity), where the gray curve is from the bare Au(111) marked by black cross in (a). Blue, red and green spectra are from the 7-aGNR and 14-aGNR edges and the junction region, respectively. Nine resonant peaks are labeled with 1–9, together with the gray dashed line for the Au SS. (c) Color-coded real space map of the band profile in the 7–14 aGNR HJ based on 50 log(dI/dV) curves along the red lines in (a). The band alignments are marked with black dashed lines. (d) The simulated local density of states (LDOS) map along the staircase edges (shadow regions) as illustrated in the atomic structure with a tip height of 3 Å above the carbon plane. The highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) in the 7-aGNR and key states in the 14-aGNR are marked. The pink dashed line marks the junction, while the white line marks the Fermi level (EF).

We now examine the electronic nature of the 7–14 aGNR HJs by measuring the local density of

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states (LDOS) with scanning tunneling spectroscopy (STS). The tunneling spectra, dI/dV were acquired with varying sample bias voltages Vs at different locations as marked in Figure 3a and displayed in Figure 3b on a logarithmic scale. The dI/dV spectrum (gray) of the bare Au(111) surface is shown as a reference, where the Au surface state (SS) appears at –0.45 eV. The Au SS is still visible on GNRs but upshifted to –0.27 eV due to modified boundary conditions.15 The blue curves taken at the 7-aGNR edge show clear peaks at Vs = –0.89 V (labeled as State 1) and 1.73 V (State 2), correspondingly to the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), indicating a bandgap of 2.62 eV, consistent with previous reports.15, 16, 19 At interface sites, the green curves display five distinct peaks at Vs = –1.15, –0.13, 0.19, 0.88 and 1.60 V, labeled as States 3–7 respectively. The peak positions of States 3 and 7 shift gradually away from those of States 1 and 2, respectively, toward lower energy. States 4–6 appear inside the gap of the 7-aGNR. At the edge of the 14-aGNR (red curves), two additional peaks labeled as States 8 and 9 are observed at Vs = –1.25 V and 1.35 V, in addition to States 5 and 6 (also existing at the interface). State 4 is convoluted with Au SS in the 14-aGNR region, but visible at the interface when the SS is suppressed. All these resonant states (States 3–7) originate either from the 7 or the 14-aGNR and no localized defect states are observed (Figure S7), indicating a seamless interface. However, some states, such as States 5 and 6, have higher intensity at the staircase corners, which may originate from the unique termini of the partial zigzag edge.30 Moreover, spatial variation in the electronic potential near the corner can also lead to a higher LDOS contrast between the edges and the interior of GNR.31 To visualize the band alignments at the 7–14 aGNR HJs, we plot the spatial distributions of different electronic states in real space in Figure 3c, where the intensity of different states is color-coded with 50 STS curves acquired across the junction along the staircase edges (red solid 9


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lines in Figure 3a). States 1 and 2 are the HOMO and LUMO of the 7-aGNR segment, respectively. States 4 and 5 correspond to the HOMO and LUMO of the 14-aGNR segment, which has a small bandgap of 0.32 eV, as expected from theoretical calculations.24, 32 The above band alignment is corroborated by first-principles DFT calculations. Figure 3d shows the calculated spatial distribution of the LDOS along the 7–14 aGNR HJ with Gaussian broadening (the mapping without broadening is shown in Figure S8). The observed resonant peaks (States 3–7) are captured by the simulations, although some predicted weak states do not show clearly in experimental dI/dV curves. In particular, a wide bandgap (~1.7 eV) of the 7-aGNR and the quasi-metallic nature of the 14-aGNR with a small bandgap of ~0.2 eV are reproduced by the calculations. Similarly to experimental observations in Figure 3c, some states have enhanced LDOS at the interface, but they are not completely localized at the interface and are present in the 14-aGNR as well (Figure S8 and S9). Notably, we also found that the electronic states from the 7-aGNR and 14-aGNR segments can directly extend into each other (Figure S8), owning to the native sp2 carbon bonding across the interface and orbital hybridization between the two segments. Such a seamless HJ facilitates charge transfer across the junction,1 and thus the quasi-metallic 14-aGNR offers an optimal electrical contact for the wide-gap semiconductor 7-aGNR. The HJ can extend to become a more complex 7–14–21 aGNR staircase. Along the straight edge of a 7–14–21 aGNR HJ (the red solid line in Figure 4a), we acquired 40 STS spectra and compared them with that of the bare Au(111) surface (gray) in Figure 4b. The states measured across the 7–14 aGNR interface are consistent with those shown in Figure 3, although with slight variations of the peak positions due to the confinement effect of the finite junction length. In the 21-aGNR region (purple curves) three main peaks are identified at Vs = –1.39 V (State 10), 0.68 V (State 11), and 1.36 10


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V (State 9’). According to a recent STM study on the 21-aGNR, a bandgap of about 0.7 eV with the HOMO and LUMO at about –0.1 eV and 0.6 eV, respectively, is expected.32 Indeed, the State 11 (Vs = 0.68 V) is observed and can be assigned to the LUMO of the 21-aGNR, while the HOMO is convoluted with the Au SS. Here, the gap for the 21-aGNR should be in the range of 0.68 – 0.95 eV. The bandgap of the 14-aGNR (~0.3 eV) is smaller than those of the 7 (~2.6 eV) and 21-aGNRs (~0.7 eV), and thus the 14-aGNR segment can host quantum dot states in the 7–14–21 aGNR HJ (Figure S10).30, 31, 33 Figure 4c shows the color-coded real-space map of electronic states. By avoiding the staircase corners, the spectra acquired along the straight edge of the HJ do not exhibit strong enhancement at the interfaces (Figure S9), and the band evolutions are more clearly presented. This data directly shows that the State 5’/5 has the contribution of the LDOS from the bulk of the 14-aGNR, consistent with Figure 3. The calculated DFT band alignments between the 7, 14, 21 and 42-aGNRs are shown in Figure 4d, with energy levels aligned to the vacuum potential. While the 7-aGNR shows a big bandgap of 1.67 eV, the 14, 21 and 42-aGNRs all have small gaps with HOMO and LUMO levels within the band gap of the 7-aGNR. Note, due to the general tendency of bandgap underestimation in DFT,34 the calculated gaps are smaller than those in experiment.

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Figure 4. Band alignments in the 7–14–21 aGNR heterostructure. (a) STM image of a 7–14–21 aGNR HJ acquired at Vs = –2.0 V, It = 100 pA. Scale bar, 2 nm. (b) The log(dI/dV) curves acquired along the red line in (a) (equally vertically shifted for clarity). A gray curve from the bare Au(111) surface is shown as reference. Apparent resonant peaks are labeled as States 1’ (Vs = –0.90 V) and 2’ (1.73 V) in the 7-aGNR, while States 5’ (0.33 V), 6’ (0.83 V), 8’ (–1.25 V), and 9’ (1.36 V) in the 14-aGNR, and States 10 (–1.39 V) and 11 (0.68 V) in the 21-aGNR. (c) Color-coded real space map of the band profile. The band alignments between different states are marked with dashed black lines. The pink dashed lines mark the junctions while the white dashed line marks the EF. The HOMO and LUMO in the 7-aGNR and LUMOs in the 14 and 21-aGNR are marked, while the HOMOs are convoluted with the Au SS. (d) Calculated band structures of the 7, 14, 21 and 42-aGNRs, with the bandgaps marked. The energy levels are all aligned to the vacuum potential.

In summary, these results indicate that GNRs with variable widths and coherent heterostructures can be realized by using only one molecular precursor. While the 7-aGNR is a large-gap

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semiconductor, the conjugated wide GNRs are either quasi-metallic or small-gap semiconductors with limited DOS at the EF, similarly to the 2D metals.5 This is different from previous reports where type-I31 and type-II18 semiconducting HJs were synthesized by using two different precursors. The lack of strongly localized interfacial states is highly desirable for making a good electrical contact to the semiconductor materials, as verified in 2D heterojunctions between the metallic 1T or 1T’-phase and the semiconducting 2H-phase transition metal dichalcogenides (MoS2 and MoTe2) where Ohmic contacts were realized.6, 7 Our staircase HJs with atomically controlled interfaces could provide a simple graphitic nanocircuitry, as illustrated in Figure S11. The wide quasi-metallic GNRs branches in the HJs act as electrical contacts to the 7-aGNR where the barriers can be greatly reduced by back-gate tuning.35 Similar staircase structures are also likely to form using other nanoribbon families, for example the recently achieved three,36 five,25 nine26 and thirteen37 carbon wide aGNRs. The graphitic structures can be transferred from the substrates that they grow on38 to form more complex nanocircuits, and the long and wide branches of the heterojunctions facilitate their integration with conventional electrodes. Therefore, the staircase HJs with atomically controlled seamless interfaces provide a promising interconnect to the semiconductor channel material, which would avoid Fermi-level pinning and a high Schottky barrier.

METHODS. Synthesis of GNR heterojunctions. The Au(111) single crystal was cleaned by the repeated cycles of Ar+ sputtering and annealing to 740 K. DBBA molecules with a purity of 98.7% were degassed at 450 K overnight in a Knudsen cell, then evaporated at 485 K for 20 s from the cell, at a substrate temperature of 470 K. The sample was subsequently annealed at 470 K and 670 K for 30 min, respectively, to induce polymerization and cyclodehydrogenation for the formation of the 13


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7-aGNRs, similarly to our previous report.23 Then the samples were further annealed to 770 K or 870 K for 30 min to promote conjugation and form wide aGNRs and the heterostructures. STM measurements. The STM characterizations were performed with a home-made variable temperature system at 105 K under ultrahigh vacuum conditions with clean commercial PtIr tip. All STM images were acquired in a constant-current mode. The dI/dV spectra were recorded using a lock-in amplifier with a sinusoidal modulation (f = 1000 Hz, Vmod = 10 mV) by turning off the feedback loop-gain. The polarity of the applied voltage refers to the sample bias with respect to the tip. Raman measurements. The Raman spectra were measured in a custom micro-Raman setup using a continuous wave solid-state laser (Excelsior, Spectra-Physics, wavelength 532 nm) and a 100× microscope objective with NA (numeric aperture) 0.9 (beam spot on the samples was ~1 µm). All measurements were carried out under a microscope in backscattering configuration. The scattered Raman light was analyzed by a spectrometer (Spectra Pro 2300i, Acton, f ~ 0.3 m) that was coupled to a microscope and equipped with a 1800 groves/mm grating and a CCD camera (Pixis 256BR, Princeton Instruments). Pure gold substrates were used to measure and subtract the photoluminescence background from all Raman spectra. Computational details. To obtain the electronic properties of aGNRs and their heterojunctions, plane-wave density functional theory (DFT) calculations were performed using the VASP package,39 projector-augmented-wave (PAW) pseudopotentials, and the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional.40 The aGNRs are periodic in the x direction with the lattice constant ~4.26 Å. Vacuum spacings of 16 Å in the non-periodic directions (y and z) were used to decouple

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periodic images. In the unit cell of aGNRs, 24×1×1 k-point sampling was used. The 7–14 aGNR HJ was modeled by 12 unit cells of the 7-aGNR and 12 unit cells of the 14-aGNR, with a total length of ~102.24 Å. It is long enough that each aGNR segment can recover bulk properties. A single Г point was used for the k-point sampling. Atomic structures were relaxed until residual forces were below 0.02 eV/Å, using the plane-wave cutoff energy of 400 eV. For the 7–14 aGNR HJ, the calculated LDOS map is based on extraction of the partial charge density from VASP at the specific positions and energies. The map was calculated at a height of 3 Å above the nanoribbon plane, as the STM tip is above the sample surface. Gaussian broadening of the position (half width at half maximum, HWHM = 5 Å) and energy (HWHM = 0.1 eV) was then applied. For the computation of vibrational properties of aGNRs, higher precision settings are required. The local density approximation (LDA) with PAW pseudopotentials was adopted with the energy cutoff set at 500 eV. All atoms were relaxed until the residual forces were below 0.001 eV/Å. In the finite difference scheme, the Hellmann-Feynman forces in a (3×1×1) supercell were computed by VASP for both positive and negative atomic displacements (δ = 0.03 Å), and used in the PHONON software41 to construct the dynamical matrix, whose diagonalization provides phonon frequencies and eigenvectors (i.e., the vibration patterns).

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website. Large-area STM images with different ribbon coverages after annealed at 770 K; DFT simulations of the width-dependent vibration modes in aGNRs; Staircase corner induced LDOS enhancements; STM images and dI/dV curves of wide GNRs; dI/dV mapping of the 15


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7–14 and 7–14–21 aGNR staircase (PDF) AUTHOR INFORMATION Corresponding Author Email: [email protected]; [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This research was conducted at the Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility. The electronic characterization was funded by ONR grants N00014-16-1-3213 and N00014-16-1-3153. The simulation work at NCSU was supported by DOE DE-FG02-98ER45685. The supercomputer time was provided by NSF grant ACI-1615114 at the National Center for Supercomputing Applications (NSF OCI-0725070 and ACI-1238993) and by DOE at the Oak Ridge Leadership Computing Facility and at the National Energy Research Scientific Computing Center. L. L. was supported by Eugene P. Wigner Fellowship at the Oak Ridge National Laboratory and by the Center for Nanophase Materials Sciences. Part of phonon calculations were performed using the resources of the Center for Computational Innovation at Rensselaer Polytechnic Institute. REFERENCES 1.

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