Graphene Nanoribbon Dielectric Passivation ... - ACS Publications

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Graphene Nanoribbon Dielectric Passivation Layers for Graphene Electronics Nobuhiko Mitoma, Yuuta Yano, Hideto Ito, Yuhei Miyauchi, and Kenichiro Itami ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00767 • Publication Date (Web): 12 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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ACS Applied Nano Materials

Graphene Nanoribbon Dielectric Passivation Layers for Graphene Electronics Nobuhiko Mitoma,†,‡,§ Yuuta Yano,†,§ Hideto Ito,†,‡,* Yuhei Miyauchi,†,‡,⊥,*

and Kenichiro Itami†,‡,¶,*

†

‡

Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan

JST-ERATO Itami Molecular Nanocarbon Project, Nagoya University, Nagoya 464-8602, Japan ⊥Institute

¶Institute

of Advanced Energy, Kyoto University, Uji 611-0011, Japan

of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Nagoya 464-8602, Japan

ACS Paragon Plus Environment

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ABSTRACT Charged-impurity scattering is a serious problem that hinders the electrical properties of graphene. Towards large-scale and/or flexible graphene-based electronics, there is a strong demand for a high-κ dielectric layer, which reduces charged-impurity density and screens the impurity scattering that passivates the graphene. We herein demonstrate that the structurally precise and soluble graphene nanoribbons (GNRs) act as excellent dielectric passivation layers. The wide-gap GNRs, synthesized through annulative π-extension polymerization, were selectively and stably fixed onto graphene via a simple drop-casting method. The carrier mobility of ~30-nm-thick GNR-adlayer-deposited graphene was approximately twice that in its original state. Electrical transport and Raman spectroscopic measurements revealed that the deposition of the GNR dielectric passivation layers reduced the charge puddles. These results suggest that the GNR adlayers prevent graphene from the oxygen/water redox couple adsorption and lift the graphene up from the underlying SiO2 substrate via strong π–π and CH–π interactions. Additionally, the relatively high dielectric constant (~5.2) of the GNRs contributes to the increased screening effect. All these effects lead to a reduced impurity scattering, which increases carrier mobility.

KEYWORDS: graphene, graphene nanoribbon, self-assembly, charged impurity, dielectric screening


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INTRODUCTION The physical properties of graphene are extremely sensitive to the surrounding environment because it is only one atom thick and has the highest surface-to-volume ratio among solids. A sheet of graphene cannot stand by itself; therefore, it requires a supporting substrate. Silicon substrates with thermal oxide layers (SiO2/Si) have been widely used since the first graphene experimental isolation has been performed in 2004.1 A well-known but serious problem is that the intrinsic electrical properties of graphene are hindered on SiO2/Si substrates by charged impurities, polar phonons, and substrate-stabilized wrinkles.2-9 Among these, the impact of charged impurities is quite severe.10 In thin-film electronics, a general approach to minimize the charged-impurity scattering is to use substrates and/or passivation layers with high dielectric constants (κ) to screen the Coulomb potential, as well as ensuring a low charged-impurity density. Hexagonal boron nitride with fewer charged impurities11 or graphene structures suspended in a vacuum12 has been used to shed light on the intrinsic electrical properties of graphene. Ferroelectric oxide substrates are also used to screen the impurity scattering.13 Additionally, organic passivation layers in graphene electronics have been proposed as an alternative to overcome the impurity scattering and to address the scalability and flexibility problems. However, the κ values of typical organic solids, which are important values that determine the ability to screen charged impurities, are low (ca. 2–3).14 Thus, a molecular dielectric with a higher κ compatible with graphene is highly desirable. Graphene nanoribbons (GNRs)15 are ideal materials for this purpose. The band-gap energy of GNRs is tunable via the precise control for their width and edge structure.16,17 Narrow GNRs with appropriate edge structures act as excellent high-κ dielectrics on graphene.18 To utilize GNRs as a dielectric passivation layer, the simplest and most cost-effective approach is to apply solvent-soluble


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GNRs on graphene using a drop-casting method. However, GNRs fabricated through lithographic cutting16 or those synthesized on metal surfaces19 are not suitable for drop casting. Thus, structurally precise, narrow, and soluble GNRs synthesized by solution-phase reactions are the most favorable. Some pioneering studies20,21 demonstrated that soluble GNRs can be synthesized through a solution-phase synthesis using multiple reaction steps. However, many reaction steps are not cost effective for large-scale applications, which means that the development of a simple synthetic route is in high demand. In this paper, we demonstrate that GNRs with precisely controlled structures function as excellent high-κ dielectric passivation layers for the improvement of the electrical properties of graphene on SiO2/Si substrates. Wide-gap and soluble GNRs of ~3 nm in width, ~140 nm in length, and with a cove-edged structure (Figure 1) were synthesized using the annulative πextension (APEX) reactions developed for the efficient, rapid, and cost-effective bottom-up synthesis of polycyclic aromatic hydrocarbons, and nanographene.22-24 We found that the GNRs with the precisely designed structure formed a self-assembled dielectric passivation layer that improved the electrical properties of graphene via a simple drop-casting method. Graphene with the GNR adlayer (GNRs/Gr) shows increased electrical transport properties that suggest fewer charged impurities, less tensile strain, and improved charge-carrier mobility, as confirmed by Raman spectroscopy. Detailed analyses of the results suggest that the GNR layer separates the graphene from extrinsic sources of charged impurities and screens the Coulomb potential that causes unwanted charge-carrier scattering.


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Figure 1. (a) Synthesis of cove-type GNR 2 by the APEX reaction of silicon-bridged phenanthrene 1. (b) Molecular modeling of a 40-mer of cove-type GNR 2, which consists of central phenanthrene rings (blue) and peripheral long alkyl chains (white and gray). Indicated lengths are estimated values of GNR 2 with Mn = 1.55 × 105 (n = 288). (c) Normalized intensity of UV-visible absorption (gray) and fluorescence (blue) spectra of GNR 2 dissolved in chloroform. The excitation wavelength for fluorescence spectroscopy was 310 nm.

RESULTS AND DISCUSSION Figure 1a shows the synthetic scheme of GNRs that have the repeating cove-regions and solubilizing long alkyl chains (Figure 1b). Applying the APEX reaction to the silicon-bridged phenanthrene 1 in the presence of Pd(OCOCF3)2 (1.0 equiv), AgSbF6 (2.0 equiv), and o-chloranil (2.0 equiv) in 1,2-dichloroethane at 80 °C, we successfully synthesized the cove-type GNR 2 (see Experimental Section and Supporting Information for full details). The UV-visible absorption spectrum of GNR 2 in chloroform revealed an optical band gap of ~3.0 eV (Figure 1c). The fluorescence peak maximum is located at 474 nm (Figure 1c), which is at a longer


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wavelength than that of a previously synthesized nanographene (447 nm)22 due to the extension of π-conjugation in GNR 2. Figure 2a and 2b show optical microscopy images of the same graphene FET before and after the deposition of GNR 2 (simply referred to as GNR hereafter), respectively. The difference in color between the original graphene and the SiO2/Si substrate is very small (Figure 2a), and it is much clearer after deposition of the GNR layer (Figure 2b). Atomic force microscopy (AFM; DimensionFastScan; Bruker, Billerica, MA, USA) was performed to clarify how the GNRs adsorbed onto the graphene. The GNRs adsorbed preferably on the graphene, and a thin GNR layer (ca. 1–2-nm thick) remained firmly attached to the graphene even after washing with 1,2,4trichlorobenzene, indicating the existence of strong π–π and CH–π interactions between the GNRs and the graphene (Figures S9 and S10). GNRs were not adsorbed onto the underlying SiO2 layer, which was probably due to the weaker interactions between GNRs and the SiO2. The GNRs form a characteristic trigonal pattern on graphene, which reflects the symmetry of the graphene lattice and supports the existence of π–π interactions (Figures S8–S10). We often found that the GNRs were aligned parallel (Figure 2c) or perpendicular (Figure 2d) to the graphene edges. The mechanical exfoliation of graphene yields crystallographically oriented edges, i.e., zigzag and armchair edges.25 Thus, the parallel or perpendicular alignment of GNRs to the graphene edges, shown in Figure 2c and 2d, respectively, can be the most stable stacking manner in graphite, i.e., AB stacking (Figure 2e).


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Figure 2. Optical microscopy images of (a) a pristine graphene FET and (b) a GNRs/Gr FET made on the identical device. AFM phase images of GNRs/Gr aligned (c) parallel and (d) perpendicular to the graphene edges. (e) The suggested AB stacking manner of the GNRs/Gr. (f) Raman spectra of pristine graphene (black) and a ~30-nm thick GNRs/Gr (red). The spectra have been stacked above each other for clarity. (g) The wavenumber plots of the 2D peak versus G peak in the pristine graphene (black) and the GNRs/Gr (red). Solid and dotted lines are axes to represent hole density and tensile strain. (h) Histogram of the 2D/G peak intensity ratio for the pristine graphene (black) and the GNRs/Gr (red). These data were obtained from an identical graphene flake before and after GNR deposition.

Raman scattering spectroscopy using a 532 nm laser was performed (inVia confocal Raman microscope; Renishaw, Wotton-under-Edge, UK) to evaluate any changes in the physical


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properties of the graphene caused by the GNR deposition. Figure 2f showed the Raman G (ca. 1588–1598 cm−1) and 2D (~2682 cm−1) bands of pristine graphene, and a ~30-nm thick GNRs/Gr whose values were measured on an identical graphene flake before and after GNR deposition. Lorentzian-shaped 2D bands confirmed that both samples possessed the characteristics of single-layer graphene.26 The Raman D (~1340 cm−1) band was not discernible before and after GNR deposition, indicating that no covalent bonds were formed between the adsorbed GNRs and graphene. A shift in the G wavenumber, and an increase in the 2D/G peak intensity ratio were observed after GNR deposition. Figure 2g shows the wavenumber correlation plot of the G and 2D bands. The observed wavenumber shifts of the G peaks suggest there were changes in the charge-carrier density (solid lines) and tensile strain (dotted lines).27,28 Based on the detailed analysis, a reduction in the hole doping and the tensile strain were deduced from shifts in the ~30-nm thick GNRs/Gr to be ~6.9 × 1012 cm−2 and ~0.2% from its original state, respectively (see the Experimental Section). These changes could be caused by the efficient lift-up and the planarization of the underlying graphene away from an atomically rough SiO2/Si surface.14 This local suspension of graphene could reduce hole doping from the substrate and it was supported by the increased 2D/G peak intensity ratio and the decreased full width at half maximum of 2D band (Figure 2f, 2h). Additionally, the GNR adlayer was expected to prevent adsorption of the oxygen/water redox couple in the air; thus, hole doping of graphene, generally observed in the ambient air, could also be reduced. The electrical properties of graphene and GNRs/Gr were measured using a semiconductor parameter analyzer (4200-SCS; Keithley Instruments, Solon, OH, USA) in the ambient air. Figure 3a shows the output curves of the graphene and the GNRs/Gr measured at charge-neutral points. Linear relationships confirmed that ohmic contacts were formed between the graphene


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channel and Au/Cr electrodes. Note that resistivity of the GNR thin film itself was higher than 108 Ωm within the applied gate-source voltage (VGS) used in the present study (Figures 3a and S11). Figure 3b showed the transfer curves whose VGS was swept from −40 to 80 V at temperatures (T) ranging from 30 to 90 °C (for the curves at intermediate temperatures, see Figure S12). Although previous studies reported that sample heating procedure more than ~250 °C improves electrical properties of graphene FETs prepared on SiO2,6,8 our experiments were performed under much milder conditions (

Graphene Nanoribbon Dielectric Passivation ... - ACS Publications

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