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Synthesis of Monolayer Hexagonal Boron Nitride on Cu Foil using Chemical Vapor Deposition

• Authors: K. K. Kim, A. Hsu, X. Jia, S. M. Kim, Y. Shi, M. Hofmann, D. Nezich, J. F. Rodriguez-Nieva, M. Dresselhaus, T. Palacios, J. Kong
• Sponsorship: NSF, ONR GATE-MURI

Hexagonal boron nitride (h-BN) is very attractive for many applications, particularly as a protective coating, dielectric layer/substrate, transparent membrane, or deep ultraviolet emitter. In this work, we carried out a detailed investigation of h-BN synthesis on Cu substrate using chemical vapor deposition (CVD) with two heating zones under low pressure (LP). Previous atmospheric pressure (AP) CVD syntheses were able to obtain only a few layers of h-BN without a good control on the number of layers ^{[1] [2]} .  In contrast, under LPCVD growth, monolayer h-BN was synthesized, and time-dependent growth was investigated (Figure 1).  It was also observed that the morphology of the Cu surface affects the location and density of the h-BN nucleation (Figure 2).  Ammonia borane, which is easily accessible and more stable under ambient conditions than borazine, is used as a BN precursor. The h-BN films are characterized by atomic force microscopy, transmission electron microscopy and electron energy loss spectroscopy analyses. Our results suggest that the growth here occurs via surface-mediated growth, which is similar to graphene growth on Cu under LP. These atomically thin layers are particularly attractive for use as atomic membranes or dielectric layers/substrates for graphene devices ^[3] .



Figure 1: Monolayer h-BN characterization. (a) Optical image of h-BN on 300 nm SiO₂/Si. White dotted line indicates the border between h-BN and SiO₂. The inset displays a photograph of h-BN film on a quartz substrate (red dotted area). (b) Optical absorption spectra of a monolayer h-BN, monolayer graphene and a quartz substrate (as a reference). (c) TEM image of the folded edge of a monolayer h-BN.



Figure 2: Effect of copper surface morphology on h-BN growth. SEM images of (a) an unpolished copper foil and (b) a polished copper foil after h-BN growth for 10 min with T₁=70 ^oC. (c)-(d) AFM images of the unpolished copper foil after growth: (c) height and (d) phase images. (e) Height profile along the red dotted in (c) and (d).




1. Y. Shi, C. Hamsen, X. Jia, K. K. Kim, A. Reina, M. Hofmann, A. L. Hsu, K. Zhang, H. Li, Z.-Y. Juang, M. S. Dresselhaus, L.-J. Li, and J. Kong, “Synthesis of few-layer hexagonal boron nitride thin film by chemical vapor deposition,” Nano Letters, vol. 10, pp. 4134-4139, Oct. 2010. [↩]
2. L. Song, L. Ci, H. Lu, P. B. Sorokin, C. Jin, J. Ni, A. G. Kvashnin, D. G. Kvashnin, J. Lou, B. I. Yakobson, and P. M. Ajayan, “Large scale growth and characterization of atomic hexagonal boron nitride layers,” Nano Letters, vol. 10, pp. 3209-15, Aug. 2010. [↩]
3. K. K. Kim, A. Hsu, X. Jia, S. M. Kim, Y. Shi, M. Hofmann, D. Nezich, J. F. Rodriguez-Nieva, M. Dresselhaus, T. Palacios, and J. Kong, “Synthesis of monolayer hexagonal boron nitride on Cu foil using chemical vapor deposition,” Nano Letters, vol. 12, pp. 161-166, Jan. 2012. [↩]

Graphene as Transparent Conducting Electrodes in Organic Photovoltaics: Studies in Graphene Morphology, Hole Transporting Layers, and Counter Electrodes

• Authors: H. Park, P. R. Brown, V. Bulović, J. Kong
• Sponsorship: Eni S.p.A, Fannie and John Hertz Foundation, NSF

Due to the remarkable physical properties of graphene, applications in various areas such as transistors ^[1] , chemical sensors ^[2] , and logic devices ^[3] have been explored; a variety of proof-of-concept devices have been demonstrated.  In this work, organic photovoltaics (OPV) with graphene electrodes are constructed so that the effects of graphene morphology (Figure 1), hole transporting layers (HTL) (Figure 2), and counter electrodes are presented.  One of the challenges in the integration of graphene in OPV is the incompatibility between the graphene electrode and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transport layer (HTL) which significantly increases the device failure rate ^[4] .  When hydrophilic PEDOT:PSS is spin-coated onto graphene, it is difficult to achieve uniform and conformal coating due to the hydrophobic nature of the graphene surface, i.e., lower surface free energy.  Instead of the conventional PEDOT:PSS HTL, an alternative transition metal oxide HTL (molybdenum oxide (MoO₃)) is investigated to address the issue of surface immiscibility between graphene and PEDOT:PSS.  The graphene films considered here are synthesized via low-pressure chemical vapor deposition (LPCVD) using a copper catalyst, and experimental issues concerning the transfer of synthesized graphene onto the substrates of OPV are discussed.  The morphology of the graphene electrode and HTL wettability on the graphene surface are shown to play important roles in the successful integration of graphene films into the OPV devices.  The effect of various cathodes on the device performance is also studied.  These factors (i.e., suitable HTL, graphene surface morphology and residues, and the choice of well-matching counter electrodes) will provide better understanding for utilizing graphene films as transparent conducting electrodes in future solar cell applications.



Figure 1: AFM images of graphene morphology of different numbers of layers: (A) 3 layers and (B) 1 layer. Cross-sectional profiles of dotted regions are shown below. Much denser concentration of wrinkles on 3-layered graphene films are observed compared to a single-layer graphene sheet. The surface of stacked layers is thus much rougher than the single-layer sheet with a rms roughness more than 3-fold higher: 2.0 nm for 3 layers and 0.6 nm for 1 layer.



Figure 2: Current density vs. voltage characteristics of graphene OPV devices with PEDOT:PSS and MoO₃ hole transporting layers with varying MoO₃ HTL thicknesses (20-40 nm) under simulated AM 1.5G illumination at 100 mW/cm². By utilizing the thermally evaporated MoO₃ HTL, the wetting of HTL on graphene surface can be improved compared to the conventional PEDOT:PSS HTL.




1. Y. M. Lin, K. A. Jenkins, A. Valdes-Garcia, J. P. Small, D. B. Farmer, and P. Avouris, “Operation of graphene transistors at gigahertz Frequencies,” Nano Letters, vol. 9, no. 1, pp. 422-426, 2009. [↩]
2. P. K. Ang, W. Chen, A. T. S. Wee, and K. P. Loh “Solution-gated epitaxial graphene as pH sensor,” Journal of the American Chemical Society, vol. 130, no. 44, pp. 14392-14393, 2008. [↩]
3. R. Sordan, F. Traversi, and V. Russo, “Logic gates with a single graphene transistor,” Applied Physics Letters, vol. 94, no. 7, p. 073305, 2009. [↩]
4. H. Park, J. A. Rowehl, K. K. Kim, V. Bulović, and J. Kong, “Doped graphene electrodes for organic solar cells,” Nanotechnology, vol. 21, no. 50, p. 505204, 2010. [↩]

2D Crystals for Ubiquitous Electronics

• Authors: H. Wang, A. Hsu, L. Yu, K. K. Kim, Y.-H. Lee, Y. Shi, J. Kong, T. Palacios
• Sponsorship: SRC/FCRP MSD, ONR GATE-MURI, Institute for Soldier Nanotechnologies, Army Resarch Laboratory

Two-dimensional crystals, including graphene, hexagonal boron nitride, and transition metal dichalcogenides (TMD) materials, have outstanding properties for developing the next generation of electronic devices ^{[1] [2]} . Graphene is the first 2D crystal to attract attention. The symmetry of its honeycomb lattice structure confers on it unique transport properties. The advantages of graphene for radio-frequency (RF) applications derive in part from its high electron and hole mobility, which can exceed 200,000 cm²/Vs at T=5 K and 100,000 cm²/Vs at T=240 K ^{[3] [4]} , the highest ever reported for any semiconductor. Moreover, graphene is a zero-bandgap material in which the conduction and valence bands touch at a point called the Dirac point. In addition to zero bandgap, the density of states in graphene is zero at the Dirac point and increases linearly for energies above and below it, which allows for carrier modulation. The carriers in graphene are confined to a one-atom-thick layer, allowing unprecedented electrostatic confinement and making graphene flexible and transparent. The lack of bandgap can be compensated for by integrating it with other 2D materials such as MoS₂from the TMD family ^{[5] [6]} . MoS₂ shares many of graphene’s advantages for electronic applications; its 1.8 eV bandgap makes it ideal for building logic circuits to complement graphene.

This project demonstrates important building blocks for future integrated circuits based on 2D-materials on flexible substrates such as plastics, paper and textiles. These basic analog building blocks include ambipolar frequency multipliers ^{[7] [8]} , graphene RF mixers ^[9] , graphene oscillators, and graphene phased shift keying devices, plus technology for fabricating flexible devices and circuits on plastic substrates. We construct integrated logic circuits based on few-layer MoS₂, including an inverter, a NAND gate, a memory device, and a ring oscillator. Prototypes of these building blocks move towards new technologies that seamlessly integrate electronics into objects of daily life, from plastic and paper cups with integrated temperature sensors and clothing with embedded RF antennas to smart contact lenses that communicate with cell phones to display relevant information to the wearer.



Figure 1: (a) Schematic of the graphene FET device structure. (b) ambipolar IDS-VGS characteristics of the fabricated GFET. (c) SEM image of the device. (d) SEM image of the T-shape gate of a self-aligned graphene transistor.



Figure 2: (a) Application circuit of graphene frequency multipliers. (b) input (red) and output (black) signals of 6-GHz graphene frequency multiplier. (c) output signal spectrum of graphene RF mixer. (d) output spectrum of a graphene electrical oscillator.




1. A. K. Geim, “Graphene: Status and prospects,” Science, vol. 324, no. 5934, pp. 1530-1534, June 2009. [↩]
2. A. H. C. Neto, et. al., “The electronic properties of graphene,” Rev. Mod. Phys., vol. 81, pp. 109–162, 2009. [↩]
3. K. I. Bolotin, et al., “Ultrahigh electron mobility in suspended graphene,” Solid State Communications, vol. 146, no. 9, June 2008. [↩]
4. K. I. Bolotin, et al., “Temperature-dependent transport in suspended graphene,” Phys. Rev. Lett., vol. 101, 096802, 2008. [↩]
5. P. Joensen, R. F. Frindt, and S. R. Morrison, “Single-layer MoS₂” Mat. Res. Bull., vol. 21, pp. 457-461, 1986. [↩]
6. B. Radisavljevic, et. al., “Single-layer MoS2 transistors,” Nature Nanotechnology, vol. 6, pp. 147–150, 2011. [↩]
7. H. Wang, D. Nezich, J. Kong, and T. Palacios, “Graphene frequency multipliers,” IEEE Electron Device Lett., vol. 30, no. 5, May 2009. [↩]
8. H. Wang, et. al., “Gigahertz ambipolar frequency multiplier based on CVD graphene,” IEDM Tech. Digest, vol. 23, no. 6, pp. 572-575, 2010. [↩]
9. H. Wang, et. al., “Graphene-based ambipolar RF mixers,” IEEE Electron Device Lett., vol. 31, no. 9, Sept. 2010. [↩]

Jing Kong

• Title: Associate Professor
• Department: Department of Electrical Engineering and Computer Science

Collaborators

• M. Dresselhaus, EECS, MIT
• A. Chandrakasan, EECS, MIT
• T. Palacios, EECS, MIT
• V. Bulovic, EECS, MIT
• P. Jarrillo-Herrero, Physics, MIT
• M. Strano, ChemE, MIT
• K. Gleason, ChemE, MIT
• J. Zhang and Z.F.Liu, Chemistry, Peking University

Graduate Students

• H. Park, Research Assistant, EECS
• A. L. Hsu, Research Assistant, EECS
• Y. Shin, Research Assistant, MSE
• W. Fang, Research Assistant, EECS
• M. Choi, Research Assistant, EECS
• Y. Song, Research Assistant, EECS
• K. Kim, Postdoctoral Researcher
• S. Kim, Postdoctoral Researcher
• S. Jung, Postdoctoral Researcher

Support Staff

• Laura M. von Bosau

Publications

Hootan Farhat, Stephane Berciaud, Martin Kalbac, Riichiro Saito, Tony F. Heinz, Mildred S. Dresselhaus, and Jing Kong, “Observation of Electronic Raman Scattering in Metallic Carbon Nanotubes”, Phys. Rev. Lett. 107, 157401 (2011).

Kyeong-Jae Lee, Masood Qazi, Jing Kong, Anantha P. Chandrakasan, “Low-Swing Signaling on Monolithically Integrated Global Graphene Interconnects,” IEEE Trans. Electron Devices, Vol .57,  Issue 12, pp. 3418-3425, Dec 2010.

Hyesung Park,Jill A. Rowehl, Ki Kang Kim, Vladimir Bulovic, and Jing Kong, “Doped graphene electrodes for organic solar cells,” Nanotechnology, 21 505204, 6 pages, 2010.

Sreekar Bhaviripudi, Xiaoting Jia, Mildred S. Dresselhaus, and Jing Kong, “Role of Kinetic Factors in Chemical Vapor Deposition Synthesis of Uniform Large Area Graphene Using Copper Catalyst,” Nano Letters, pp. 4128–4133, September 2, 2010.

Yumeng Shi, Christoph Hamsen, Xiaoting Jia, Ki Kang Kim, Alfonso Reina, Mario Hofmann, Allen Long Hsu, Kai Zhang, Henan Li, Zhen-Yu Juang, Mildred. S. Dresselhaus, Lain-Jong Li, and Jing Kong, “Synthesis of Few-Layer Hexagonal Boron Nitride Thin Film by Chemical Vapor Deposition,” Nano Letters, pp. 4034-4039, September 2, 2010.

Ki Kang Kim, Alfonso Reina, Yumeng, Shi, Hyesung Park, Lain-Jong Li, Young Hee Lee, Jing Kong, “Enhancing the conductivity of transparent graphene films via doping”, Nanotechnology, Vol. 21, Issue: 28, Article Number 285205, 6 pages, July 2010).

Liming Xie, Shin G. Chou, Ajay Pande, Jayanti Pande, Jin Zhang, Mildred S. Dresselhaus, Jing Kong, Zhongfan Liu, “Single-Walled Carbon Nanotubes Probing the Denaturation of Lysozyme”, J. of Physical Chemistry C, Vol. 114, Issue 17, pp. 7717-7720, May 2010.

Yumeng Shi, Ki Kang Kim, Alfonso Reina, Mario Hofmann, Lain-Jong Li, Jing Kong, “Work function engineering of graphene electrode via chemical doping,” ACS Nano, vol 4, 2689-2694, 2010.

Thiele, S, Reina, A, Healey, P, Kedzierski, J, Wyatt, P, Hsu, PL, Keast, C, Schaefer, J, Kong, J, “Engineering polycrystalline Ni films to improve thickness uniformity of the chemical-vapor-deposition-grown graphene films”, Nanotechnology, vol 21, Art. No. 015601, 2010.

Doping of Graphene Devices through Chlorination

• Authors: X. Zhang, K. K. Kim, J. Kong, M. Dresselhaus, T. Palacios

Graphene, a two-dimensional honeycomb lattice of sp²-hybridized carbon atoms, has attracted tremendous interest in the scientific community. Surface functionalization is a technology to engineer its electronic properties and make it even more desirable and controllable for electronic device applications. For example, chemical functionalization of graphene, especially by hydrogen, fluorine, and chlorine is predicted to enable doping, edge passivation, and opening of its bandgap. Here, we demonstrate that exposure of graphene device to chlorine plasma in an electron cyclotron resonance (ECR) plasma etcher is an effective way to tune the Fermi level of graphene carriers towards p-type direction, without sacrificing its high conductivity.

Figure 1 compares the Raman spectroscopy of graphene before and after chlorine plasma treatment and also shows how it changes after annealing. After chlorine plasma treatment for 30 s, the D band increases substantially while the G band blue shift by about 5 cm^-1. More importantly, the D band decreases significantly after a 30-min annealing at 500 °C, which means little defect creation occurs during the plasma process.  Further transport measurement shows that the carrier concentration in graphene increases considerably. The plasma reaction process tunes the Fermi level of an intrinsic p-type CVD graphene (V_Dirac >0) towards p-type direction further. We notice that the carrier mobility reduces after plasma reaction. However, this reduction should be mainly due to the increase of hole concentration in graphene rather than defect creation, which is confirmed by the decrease of its sheet resistance and increase of conductivity (see Figure 2). In addition, XPS analysis shows that the percentage of chlorine on the surface of graphene is around 20%. This approach indicates an important way to engineer graphene properties for high-performance applications of graphene devices.



Figure 1: Raman spectroscopy of graphene before plasma treatment, after plasma treatment, and after annealing.



FIgure 2: Comparison: conductivity, carrier concentration, mobility and sheet resistance of graphene devices before and after chlorination.




Graphene Chemical Sensors on Flexible Substrates

• Authors: B. Mailly-Giacchetti, F. Pappalardo, J. Kong, T. Palacios
• Sponsorship: STMicroelectronics, Institute for Soldier Nanotechnologies, International Iberian Nanotechnology Laboratory

With its all-surface 2D structure combined with very high carrier mobility, graphene is an extremely promising candidate for high sensitivity and low noise chemical sensing ^{[1] [2]} . Graphene devices can perform electrical detection for chemical sensing in many applications, e.g.,pH monitoring in electrolytes ^{[3] [4]} , measuring blood glucose, and in-vitro recording of the electrical activity of living cells ^[5] . Integrating graphene on plastic substrates ^[6] enables fabrication of low-cost, flexible sensors.

Our group has developed the first solution-gated graphene field effect transistor (SGFET) on a polyethylene naphthalate (PEN) substrate. These devices have important advantages with respect to traditional sensors, e.g., intrinsic signal amplification, simple electronic read-out, and straightforward integration with microelectronics. Fabrication of the SGFET starts with the transferring of graphene grown by chemical vapor deposition on the PEN substrate. Metal contacts were previously evaporated on the PEN substrate. Silicone rubber-based insulation was used to protect the metal contacts and wires from any contact with the electrolyte.

As Figure 2.a) shows, the characterization of our devices in a phosphate buffer solution demonstrates good transconductance, around 1 mS.mm^-1, and carrier mobility of 300 cm².V^-1.s^-1. Transconductance is 5 times smaller than that obtained for graphene on SiO₂ which is due to the 10 times rougher surface of PEN compared to SiO₂ and also the larger amount of PMMA residues remaining on graphene when transferred onto PEN. The pH measurements have been performed using these devices by recording how the Dirac point shifts with changes in pH, as Figure 2.b) shows. Sensitivity  to 22 mV/pH has been demonstrated. In addition, long-term pH monitoring was shown for the first time in these graphene devices, as Figure 2.c) shows. Our on-going work focuses on functionalizing graphene to demonstrate new biosensors for glucose and E. coli detection on plastic substrate.



Figure 1: a) Optical image of graphene chemical sensor array on plastic substrate. Inset on bottom right corner shows detail . b) Schematic of a graphene-on-PEN solution-gated transistor.



Figure 2: Transfer characteristics of 50×1000-µm² graphene-on-PEN SGFET at constant drain-source voltage of V_ds=50 mV for different pHs. b) Dirac point shift with pH. c) Drain-source current vs. time data for pHs 9-4.




1. M. Dankerl, M. V. Hauf, A. Lippert, L. H. Hess, S. Birner, I. D. Sharp, A. Mahmood, P. Mallet, J. Veuillen, M. Stutzmann, and J. A. Garrido, “Graphene solution-gated field-effect transistor array for sensing applications,” Adv. Funct. Mater., vol. 20, pp. 3117-3124, 2010. [↩]
2. L. H. Hess, M. V. Hauf, M. Seifert, F. Speck, T. Seyller, M. Stutzmann, I. D. Sharp, and J. A. Garrido, “High-transconductance graphene solution-gated field effect transistors,” App. Phys. Letters, vol. 99, p. 033503, 2011. [↩]
3. Y. Ohno, K. Maehashi, Y. Yamashiro, and K. Matsumoto, “Electrolyte-gated graphene field-effect transistors for detecting pH and protein adsorption,” Nano Lett., vol. 9, pp. 3318-3322, 2009. [↩]
4. J. Ristein, W. Zhang, F. Speck, M. Ostler, L. Ley and T. Seyller, “Characteristics of solution gated field effect transistors on the basis of epitaxial graphene on silicon carbide,” J. Phys. D: Appl. Phys., vol. 43, no. 34, p. 345303, 2010. [↩]
5. T. Cohen-Karni, Q. Qing, Q. Li, Y. Fang, and C. M. Lieber, “Graphene and nanowire transistors for cellular interfaces and electrical recording,” Nano Lett,, vol. 10, pp. 1098-1102, 2010. [↩]
6. S. Bae, H. Kim, Y. Lee, X. Xu, J. S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. I. Song, Y. J. Kim, K. S. Kim, B. Ozyilmaz, J. H. Ahn, B. H. Hong, and S. Iijima, “Roll-to-roll production of 30-inch graphene films for transparent electrodes,” Naure. Nanotechnology, vol. 5, pp. 574-578, 2010. [↩]

Graphene Infrared Photodetectors

• Authors: L. Hsu, S. J. Ha, K. K. Kim, J. Kong, T. Palacios
• Sponsorship: ONR GATE-MURI, SRC/FCRP MSD, Institute of Soldier Nanotechnology



Figure 1: (a) Scanning electron microscopy (SEM) of bowtie array; (b) SEM of single element bowtie; (c) Fourier Transform infrared spectroscopy setup for characterization of graphene infrared photodetectors.

Graphene is a two-dimensional (2D) material that has attracted great interest for electronic devices since its discovery in 2004 ^[1] . With essentially the same lattice structure as an unwrapped carbon nanotube, graphene shares many of its advantages, such as having the highest intrinsic carrier mobility at room temperature of any known material ^[2] . This trait, combined with its high electron velocity and thermal conductivity, makes this carbon-based material an excellent candidate for high frequency electronic applications. Furthermore, graphene, due to its zero band gap band structure, also has a broadband optical absorption ranging from the far-infrared (λ>10 µm) all the way to the visible (λ< 532 nm) ^[3] . Standard state-of-the-art infrared focal plane arrays rely on very exotic materials systems such as InSb and HgCdTe, both of which are difficult to grow as well as difficult to integrate cheaply with silicon read out electronics ^[4] . Graphene, on the other hand, could enable a new generation of carbon-based infrared photodetectors.

Metal-graphene-metal photodetectors have been demonstrated in the visible range (532 nm) using exfoliated highly oriented pyrolytic graphite (HOPG); however, the external efficiencies of these devices are generally low (<1%) ^[5] . Graphene photodetectors may intrinsically have efficiencies ranging from 10-15%; however, poor optical coupling with the incident light reduces the overall efficiencies of these devices. Therefore, to better address these issues, we have begun fabrication of large area photodetector arrays utilizing plasmonic antenna elements to better match the incoming light to the spatial response of the graphene photodetectors designed in the infrared (Figure 1 (a) and (b)). Furthermore, using chemically vapor grown graphene, we have been fabricating large array photodetector arrays (1 cm x 1 cm) to better extract power from the incident electromagnetic radiation.  The setup for characterizing the optical properties of these photodetectors is shown in Figure 1(c).

1. K. S. Novoselov, et al., “Electric field effect in atomically thin carbon films,” Science, vol. 306, pp. 666-669, 2004. [↩]
2. Bolotin, et. al., “Ultrahigh electron mobility in suspended graphene,” Solid State Communications, vol. 146, pp. 351-355, 2008. [↩]
3. Z. Q. Li, et al., “Dirac charge dynamics in graphene by infrared spectroscopy,” Nature Physics, vol. 4, pp. 532-535, 2008. [↩]
4. A. Rogalski, “HgCdTe infrared detector material: History, status, and outlook,” Rep. Prog. Phys, vol. 68, p. 2267, 2005. [↩]
5. F. Xia, et al., “Ultrafast graphene photodetector,” Nature Nanotechnology, vol. 4, pp. 839-843, 2009. [↩]