Manipulating thermal conductance at metal-graphene contacts via chemical functionalization

Graphene-based devices have garnered tremendous attention due to the unique physical properties arising from this purely two-dimensional carbon sheet leading to tremendous efficiency in the transport of thermal carriers (i.e., phonons). However, it is necessary for this two-dimensional material to be able to efficiently transport heat into the surrounding 3D device architecture in order to fully capitalize on its intrinsic transport capabilities. Therefore, the thermal boundary conductance at graphene interfaces is a critical parameter in the realization of graphene electronics and thermal solutions. In this work, we examine the role of chemical functionalization on the thermal boundary conductance across metal/graphene interfaces. Specifically, we metalize graphene that has been plasma functionalized and then measure the thermal boundary conductance at Al/graphene/SiO(2) contacts with time domain thermoreflectance. The addition of adsorbates to the graphene surfaces are shown to influence the cross plane thermal conductance; this behavior is attributed to changes in the bonding between the metal and the graphene, as both the phonon flux and the vibrational mismatch between the materials are each subject to the interfacial bond strength. These results demonstrate plasma-based functionalization of graphene surfaces is a viable approach to manipulate the thermal boundary conductance.     

Microstructure evolution and development of annealed Ni/Au contacts to GaN nanowires

The development of Ni/Au contacts to Mg-doped GaN nanowires (NWs) is examined. Unlike Ni/Au contacts to planar GaN, current-voltage (I-V) measurements of Mg-doped nanowire devices frequently exhibit a strong degradation after annealing in N(2)/O(2). This degradation originates from the poor wetting behavior of Ni and Au on SiO(2) and the excessive void formation that occurs at the metal/NW and metal/oxide interfaces. The void formation can cause cracking and delamination of the metal film as well as reduce the contact area at the metal/NW interface, which increases the resistance. The morphology and composition of the annealed Ni/Au contacts on SiO(2) and the p-GaN films were investigated by scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS) and x-ray diffraction (XRD) measurements. Adhesion experiments were performed in order to determine the degree of adhesion of the Ni/Au films to the SiO(2) as well as observe and analyze the morphology of the film's underside by SEM. Device degradation from annealing was prevented through the use of a specific adhesion layer of Ti/Al/Ni deposited prior to the nanowire dispersal and Ni/Au deposition. I-V measurements of NW devices fabricated using this adhesion layer showed a decrease in resistance after annealing, whereas all others showed an increase in resistance. Transmission electron microscopy (TEM) on a cross-section of a NW with Ni/Au contacts and a Ti/Al/Ni adhesion layer showed a lack of void formation at the contact/NW interface. Results of the XRD and TEM analysis of the NW contact structure using a Ti/Al/Ni adhesion layer suggests Al alloying of the Ni/Au contact increases the adhesion and stability of the metal film as well as prevents excessive void formation at the contact/NW interface.     

Thermal conduction across the one-dimensional interface between a MoS<Subscript>2</Subscript> monolayer and metal electrode

The thermal conductance across the one-dimensional (1D) interface between a MoS2 monolayer and Au electrode (edge-contact) has been investigated using molecular dynamics simulations.Although the thermal conductivity of monolayer MoS2 is 2-3 orders of magnitude lower than that of graphene,the covalent bonds formed at the interface enable interfacial thermal conductance (ITC) that is comparable to that of a graphene-metal interface.Each covalent bond at the interface serves as an independent channel for thermal conduction,allowing ITC to be tuned linearly by changing the interfacial bond density (controlling S vacandes).In addition,different Au surfaces form different bonding configurations,causing large ITC variations.Interestingly,the S vacancies in the central region of MoS2 only slightly affect the ITC,which can be explained by a mismatch of the phonon vibration spectra.Further,at room temperature,ITC is primarily dominated by phonon transport,and electron-phonon coupling plays a negligible role.These results not only shed light on the phonon transport mechanisms across 1D metal-MoS2 interfaces,but also provide guidelines for the design and optimization of such interfaces for thermal management in MoS2-based electronicdevices.     

Transport in nanoribbon interconnects obtained from graphene grown by chemical vapor deposition

We study graphene nanoribbon (GNR) interconnects obtained from graphene grown by chemical vapor deposition (CVD). We report low- and high-field electrical measurements over a wide temperature range, from 1.7 to 900 K. Room temperature mobilities range from 100 to 500 cm(2)·V(-1)·s(-1), comparable to GNRs from exfoliated graphene, suggesting that bulk defects or grain boundaries play little role in devices smaller than the CVD graphene crystallite size. At high-field, peak current densities are limited by Joule heating, but a small amount of thermal engineering allows us to reach ～2 × 10(9) A/cm(2), the highest reported for nanoscale CVD graphene interconnects. At temperatures below ～5 K, short GNRs act as quantum dots with dimensions comparable to their lengths, highlighting the role of metal contacts in limiting transport. Our study illustrates opportunities for CVD-grown GNRs, while revealing variability and contacts as remaining future challenges.     

Graphene bimetallic-like cantilevers: probing graphene/substrate interactions

The remarkable mechanical properties of graphene, the thinnest, lightest, and strongest material in existence, are desirable in applications ranging from composite materials to sensors and actuators. Here, we demonstrate that these mechanical properties are strongly affected by the interaction with the substrate onto which graphene is deposited. By measuring the temperature-dependent deflection of graphene/substrate "bimetallic" cantilevers we determine strain, thermal expansion coefficient, and the adhesion force acting on graphene films attached to a substrate. Graphene deposited on silicon nitride (SiN(x)) is under much larger strain, ε(g) ～ 1.5 × 10(-2), compared to graphene on gold (Au), ε(g) < 10(-3). The thermal expansion coefficient α(g) of graphene attached to SiN(x) is found to be negative, in the range from (- 5... - 1) × 10(-6)K(-1) and smaller in magnitude than α(g) of suspended graphene. We also estimate the interfacial shear strength of the graphene/SiN(x) interface to be ～1 GPa at room temperature.     

Thermal transport across twin grain boundaries in polycrystalline graphene from nonequilibrium molecular dynamics simulations

We have studied the thermal conductance of tilt grain boundaries in graphene using nonequilibrium molecular dynamics simulations. When a constant heat flux is allowed to flow, we observe sharp jumps in temperature at the boundaries, characteristic of interfaces between materials of differing thermal properties. On the basis of the magnitude of these jumps, we have computed the boundary conductance of twin grain boundaries as a function of their misorientation angles. We find the boundary conductance to be in the range 1.5 × 10(10) to 4.5 × 10(10) W/(m(2) K), which is significantly higher than that of any other thermoelectric interfaces reported in the literature. Using the computed values of boundary conductances, we have identified a critical grain size of 0.1 μm below which the contribution of the tilt boundaries to the conductivity becomes comparable to that of the contribution from the grains themselves. Experiments to test the predictions of our simulations are proposed.     

Low-resistance ohmic contacts to SiC nanowires and their applications to field-effect transistors

We report on the electrical characterization of two ohmic contacts (Ti/Au and Ni/Au) to unintentionally doped silicon carbide nanowires (SiCNWs) using the modified transmission line model (TLM) method. Our results indicate that subsequently deposited Ni/Au ohmic contacts on SiCNWs had ～40 times lower specific contact resistances (SCRs) of 5.9 × 10(-6) ± 8.8 × 10(-6)?Ω?cm(2) compared to the values of Ti/Au ohmic contacts (2.6 × 10(-4) ± 3.4 × 10(-4)?Ω?cm(2)). We also conducted a comparison study of the electrical characteristics of top-gated SiCNW field-effect transistors (FETs) with two different ohmic contacts as used for ohmic contact studies. The electrical transport measurements on the SiCNW FET with Ni/Au ohmic contacts show much lower resistance contacts to SiC NWs and better FET performances than those for Ti/Au ohmic contact-based FETs.     

Temperature dependence of the Raman spectra of graphene and graphene multilayers

We investigated the temperature dependence of the frequency of G peak in the Raman spectra of graphene on Si/SiO2 substrates. The micro-Raman spectroscopy was carried out under the 488 nm laser excitation over the temperature range from -190 to +100 degrees C. The extracted value of the temperature coefficient of G mode of graphene is chi = -0.016 cm-1/ degrees C for the single layer and chi = -0.015 cm-1/ degrees C for the bilayer. The obtained results shed light on the anharmonic properties of graphene.     

Thermal transport in suspended and supported few-layer graphene

We report thermal conductivity (κ) measurements from 77 to 350 K on both suspended and supported few-layer graphene using a thermal-bridge configuration. The room temperature value of κ is comparable to that of bulk graphite for the largest flake, but reduces significantly for smaller flakes. The presence of a substrate lowers the value of κ, but the effect diminishes for the thermal transport in the top layers away from the substrate. For the suspended sample, the temperature dependence of κ follows a power law with an exponent of 1.4 ± 0.1, suggesting that the flexural phonon modes contribute significantly to the thermal transport of the suspended graphene. The measured values of κ are generally lower than those from theoretical studies. We attribute this deviation to the phonon-boundary scattering at the graphene-contact interfaces, which is shown to significantly reduce the apparent measured thermal conductance of graphene.     

Electronic structures of an epitaxial graphene monolayer on SiC(0001) after gold intercalation: a first-principles study

The atomic and electronic structures of an Au-intercalated graphene monolayer on the SiC(0001) surface were investigated using first-principles calculations. The unique Dirac cone of graphene near the K?point reappeared as the monolayer was intercalated by Au atoms. Coherent interfaces were used to study the mismatch and the strain at the boundaries. Our calculations showed that the strain at the graphene/Au and Au/SiC(0001) interfaces also played a key role in the electronic structures. Furthermore, we found that at an Au coverage of 3/8?ML, Au intercalation leads to a strong n-type doping of graphene. At 9/8?ML, it exhibited a weak p-type doping, indicative that graphene was not fully decoupled from the substrate. The shift in the Dirac point resulting from the electronic doping was not only due to the different electronegativities but also due to the strain at the interfaces. Our calculated positions of the Dirac points are consistent with those observed in the ARPES experiment (Gierz et al 2010 Phys. Rev. B 81 235408).     

Graphene Microbolometers with Superconducting Contacts for Terahertz Photon Detection

We report on noise and thermal conductance measurements taken in order to determine an upper bound on the performance of graphene as a terahertz photon detector. The main mechanism for sensitive terahertz detection in graphene is bolometric heating of the electron system. To study the properties of a device using this mechanism to detect terahertz photons, we perform Johnson noise thermometry measurements on graphene samples. These measurements probe the electron–phonon behavior of graphene on silicon dioxide at low temperatures. Because the electron–phonon coupling is weak in graphene, superconducting contacts with large gap are used to confine the hot electrons and prevent their out-diffusion. We use niobium nitride leads with a \(T_\mathrm {c}\approx 10\)  K to contact the graphene. We find these leads make good ohmic contact with very low contact resistance. Our measurements find an electron–phonon thermal conductance that depends quadratically on temperature above 4 K and is compatible with single terahertz photon detection.     

Electrical and thermal conductivity of low temperature CVD graphene: the effect of disorder

In this paper we present a study of graphene produced by chemical vapor deposition (CVD) under different conditions with the main emphasis on correlating the thermal and electrical properties with the degree of disorder. Graphene grown by CVD on Cu and Ni catalysts demonstrates the increasing extent of disorder at low deposition temperatures as revealed by the Raman peak ratio, IG/ID. We relate this ratio to the characteristic domain size, La, and investigate the electrical and thermal conductivity of graphene as a function of La. The electrical resistivity, ρ, measured on graphene samples transferred onto SiO2/Si substrates shows linear correlation with La(-1). The thermal conductivity, K, measured on the same graphene samples suspended on silicon pillars, on the other hand, appears to have a much weaker dependence on La, close to K～La1/3. It results in an apparent ρ～K3 correlation between them. Despite the progressively increasing structural disorder in graphene grown at lower temperatures, it shows remarkably high thermal conductivity (10(2)-10(3) W K(-1) m(-1)) and low electrical (10(3)-3×10(5) Ω) resistivities suitable for various applications.     

Nanometer scale electrode separation (nanogap) using electromigration at room temperature

Pairs of electrodes with nanometer separation (nanogap) are achieved through an electromigration-induced break-junction (EIBJ) technique at room temperature. Lithographically defined gold (Au) wires are formed by e-beam evaporation over oxide-coated silicon substrates silanized with (3-Mercaptopropyl)trimethoxysilane (MPTMS) and then subjected to electromigration at room temperature to create a nanometer scale gap between the two newly formed Au electrodes. The MPTMS is an efficient adhesive monolayer between SiO/sub 2/ and Au. Although the Au wires are initially 2 /spl mu/m wide, gaps with length /spl sim/1 nm and width /spl sim/5 nm are observed after breaking and imaging through a field effect scanning electron microscope. This technique eliminates the presence of any residual metal interlink in the adhesion layer (chromium or titanium for Au deposition over SiO/sub 2/) after breaking the gold wire, and it is much easier to implement than the commonly used low-temperature EIBJ technique which needs to be executed at 4.2 K. Metal-molecule-metal structures with symmetrical metal-molecule contacts at both ends of the molecule are fabricated by forming a self-assembled monolayer of -dithiol molecules between the EIBJ-created Au electrodes with nanometer separation. Electrical conduction through single molecules of 1,4-Benzenedimethanethiol (XYL) is tested using the Au/XYL/Au structure with chemisorbed gold-sulfur coupling at both contacts.     

Ti/SiO_2界面反应的研究

运用俄歇深度剖析和俄歇化学效应研究了Ti／SiO2的界面还原反应。结果表明，在薄膜样品的制备过程中，金属Ti可以和SiO2发生界面还原反应，形成TiSi和TiO2物种。薄膜样品在RTA处理时，Ti和SiO2的界面反应大幅度地增加，尤其是当RTA温度高于700℃时，界面反应增加得更快。700℃温度可能是Ti和SiO2界面反应的活化温度。随着反应时间的增加，界面反应也相应增加。当Ti层的厚度增加时，也有利于Ti和SiO2的界面反应。     

Thermal reliability of n-GaAs/Ti/Pt/Au Schottky contacts with thin Ti films for reduced gate resistance

The thermal stability of Ti/Pt/Au Schottky contacts on n-GaAs with Ti films 0–60 nm is investigated. The contacts with Ti films as small as 10 nm remain thermally stable with annealing up to 400°C. The changes induced by thermal treatment in the electrical characteristics of the contacts are correlated with the Rutherford backscattering and microscopic analysis of the annealed samples. It shows profuse interdiffusion and interfacial reaction with 300°C anneal for the GaAs/Pt/Au system. It has been found that introducing the Ti film between GaAs and Pt/Au, the interdiffusion of Pt and Au is also prevented. These results are useful for reducing the gate metallisation resistance of metal semiconductor field effect transistors.     

Electron and phonon transport in Au nanoparticle decorated graphene nanoplatelet nanostructured paper

Monodispersed Au nanoparticles are synthesized on the surface of exfoliated graphene nanoplatelets (GNP) in the presence of polyethyleneimine (PEI) with microwave assisted heating. A highly structured layered Au/GNP "paper" with good flexibility and mechanical robustness is prepared by vacuum assisted self-assembly. The thermal and electrical conductivity of the hybrid paper with and without the Au nanoparticles are investigated after different experimental processing conditions including thermal annealing and cold compaction. Annealing effectively decomposes and removes the adsorbed PEI molecules and improves thermal contact between Au/GNP particles, whereas cold compaction reduces porosity and induces stronger alignment of the Au/GNP within the hybrid paper. Both approaches lead to improvement in electrical and thermal conductivity. It is also found that adjacent GNP particles are electrically connected by the Au nanoparticles but thermally disconnected. It is believed that phonons are scattered at the Au/GNP interfaces, whereas electrons can tunnel across this interface, resulting in a separation of electron and phonon transport within this hybrid paper.     

SiO2/TiO2复合气凝胶的孔道结构研究

为了在常压干燥下制备高比表面积且具有多级孔道结构的SiO2/TiO2复合气凝胶,以正硅酸乙酯、钛酸丁酯为原料,利用低聚体聚合将分相平行引入到溶胶凝胶过程中,获得SiO2/TiO2醇凝胶,并通过溶剂替换技术实现气凝胶的常压干燥制备.不同硅钛比气凝胶的内部结构研究表明:合成的气凝胶是由纳米SiO2和TiO2颗粒分散复合而成的介孔块体,其中Ti—O—Ti、Si—O—Si和Ti—O—Si键相互交织.气凝胶的结构变化是分相与溶胶凝胶过程相互竞争的结果.Si含量能显著改善气凝胶的结构,当n(Ti)∶n(Si)为3∶1时,比表面积高达712.2 m2/g,平均孔径为3.36 nm;当n(Ti)∶n(Si)为1.5∶1时,复合气凝胶具有明显双连续孔道,比表面积高,同时孔状结构清晰.     

Lowering the Schottky Barrier Height by Graphene/Ag Electrodes for High‐Mobility MoS2 Field‐Effect Transistors

2D transition metal dichalcogenides (TMDCs) have emerged as promising candidates for post‐silicon nanoelectronics owing to their unique and outstanding semiconducting properties. However, contact engineering for these materials to create high‐performance devices while adapting for large‐area fabrication is still in its nascent stages. In this study, graphene/Ag contacts are introduced into MoS₂ devices, for which a graphene film synthesized by chemical vapor deposition (CVD) is inserted between a CVD‐grown MoS₂ film and a Ag electrode as an interfacial layer. The MoS₂ field‐effect transistors with graphene/Ag contacts show improved electrical and photoelectrical properties, achieving a field‐effect mobility of 35 cm² V^?1 s^?1, an on/off current ratio of 4 × 10⁸, and a photoresponsivity of 2160 A W^?1, compared to those of devices with conventional Ti/Au contacts. These improvements are attributed to the low work function of Ag and the tunability of graphene Fermi level; the n‐doping of Ag in graphene decreases its Fermi level, thereby reducing the Schottky barrier height and contact resistance between the MoS₂ and electrodes. This demonstration of contact interface engineering with CVD‐grown MoS₂ and graphene is a key step toward the practical application of atomically thin TMDC‐based devices with low‐resistance contacts for high‐performance large‐area electronics and optoelectronics.     

Physical vapor deposition of metal nanoparticles on chemically modified graphene: observations on metal-graphene interactions

The growth of metallic nanoparticles formed on chemically modified graphene (CMG) by physical vapor deposition is investigated. Fine control over the size (down to ～1.5 nm for Au) and coverage (up to 5 × 10(4) μm(-2) for Au) of nanoparticles can be achieved. Analysis of the particle size distributions gives evidence for Au nanocluster diffusion at room temperature, while particle size statistics differ clearly between metal deposited on single- and multilayer regions. The morphology of the nanoparticles varies markedly for different metals (Ag, Au, Fe, Pd, Pt, Ti), from a uniform thin film for Ti to a droplet-like growth for Ag. A simple model explains these morphologies, based only on consideration of 1) the different energy barriers to surface diffusion of metal adatoms on graphene, and 2) the ratio of the bulk cohesive energy of the metal to the metal-graphene binding energy. Understanding these interactions is important for controlling nanoparticle and thin-film growth on graphene, and for understanding the resultant charge transfer between metal and graphene.     

钛掺入对Cu/Si（100）及Cu/SiO2薄膜体系热稳定性的影响

利用简易合金靶材在Si（100）和SiO2基底上磁控溅射制备了Cu（1.42%Ti）薄膜。研究了少量钛对Cu/Si（100）和Cu/SiO2薄膜体系在573-773 K退火前后的微观组织结构以及界面反应的影响。X射线衍射分析表明,溅射态Cu（Ti）薄膜均呈现Cu（111）和Cu（200）衍射峰,而钛显著增强铜薄膜的（111）织构。对于退火态的Cu（Ti）/Si薄膜体系,由于少量钛在薄膜/基底界面处的存在,起到净化界面作用,促使Cu3Si的形成,从而降低了薄膜体系的热稳定性。但对于Cu（Ti）/SiO2薄膜体系,在773 K退火后,仍然呈现出良好的热稳定性。薄膜截面的结构形貌以及界面处俄歇谱的分析结果都充分证实了上述结果。