Sheng Liu

New materials graphyne, graphdiyne, graphone, and graphane: review of properties, synthesis, and application in nanotechnology

Plenty of new two-dimensional materials including graphyne, graphdiyne, graphone, and graphane have been proposed and unveiled after the discovery of the “wonder material” graphene. Graphyne and graphdiyne are two-dimensional carbon allotropes of graphene with honeycomb structures. Graphone and graphane are hydrogenated derivatives of graphene. The advanced and unique properties of these new materials make them highly promising for applications in next generation nanoelectronics. Here, we briefly review their properties, including structural, mechanical, physical, and chemical properties, as well as their synthesis and applications in nanotechnology. Graphyne is better than graphene in directional electronic properties and charge carriers. With a band gap and magnetism, graphone and graphane show important applications in nanoelectronics and spintronics. Because these materials are close to graphene and will play important roles in carbon-based electronic devices, they deserve further, careful, and thorough studies for nanotechnology applications.

Mechanical stabilities and properties of graphene-like aluminum nitride predicted from first-principles calculations

A graphene-like hexagonal aluminum nitride monolayer (g-AlN) is a promising nanoscale optoelectronic material. We investigate its mechanical stability and properties using first-principles plane-wave calculations based on density-functional theory, and find that it is mechanically stable under various strain directions and loads. g-AlN can sustain larger uniaxial and smaller biaxial strains than g-BN before it ruptures. The third, fourth, and fifth-order elastic constants are essential for accurately modeling the mechanical properties under strains larger than 0.02, 0.06, and 0.12 respectively. The second-order elastic constants, including in-plane stiffness, are predicted to monotonically increase with pressure while the Poisson ratio monotonically decreases with increasing pressure. g-AlN’s tunable sound velocities have promising applications in nano waveguides and surface acoustic wave sensors.

Mechanical properties and stabilities of g-ZnS monolayers

We investigate the mechanical properties and stabilities of the planar graphene-like zinc sulfide (g-ZnS) monolayers under various large strains using electronic structure calculations. The g-ZnS has a low in-plane stiffness, about 1/8 of that of graphene. The potential profiles and the stress–strain curves indicate that the free standing g-ZnS monolayers can sustain large tensile strains, up to 0.16, 0.22, and 0.19 for armchair, zigzag, and biaxial deformations, respectively. However, both the strength and flexibility are reduced compared to graphene-like zinc oxide monolayers. The third, fourth, and fifth order elastic constants are indispensable for accurate modeling of the mechanical properties under strains larger than 0.02, 0.04, and 0.08 respectively. The second order elastic constants, including in-plane stiffness, are predicted to monotonically increase with pressure, while the trend in the Poisson ratio is reversed. Our results imply that g-ZnS monolayers are mechanically stable under various large strains. The elastic limits provide a safe-guide for strain-engineering the g-ZnS based electronics.

Temperature dependence of Raman spectra of graphene on copper foil substrate

We investigate the temperature dependence of the phonon frequencies of the G and 2D modes in the Raman spectra of monolayer graphene grown on copper foil by chemical vapor deposition. The Raman spectroscopy is carried out under a 532.16xa0nm laser excitation over the temperature range from 150 to 390xa0K. Both the G and 2D modes exhibit significant red shift as temperature increases, and the extracted values of temperature coefficients of G and 2D modes are −0.101 and −0.180xa0cm−1xa0K−1, respectively, different from that of graphene on SiO2 substrate. The obtained results shed light on the anharmonic property of graphene, the complex interfacial interactions between graphene and the underlying copper foil substrate as temperature changes, and also proposes a new routine to estimate the thermal expansion coefficient of graphene on copper substrate rather than on SiO2 and SiN substrates. Furthermore, our work is instructive to study the similar temperature dependent mechanical properties, and the interfacial interactions between the other emerging two dimensional materials and their underlying substrates by temperature dependent Raman scatterings.

Off-state electrical breakdown of AlGaN/GaN/Ga(Al)N HEMT heterostructure grown on Si(111)

Electrical breakdown characteristics of AlxGa1−xN buffer layers grown on Si(111) are investigated by varying the carbon concentration ([C]: from ∼1016 to 1019 cm−3), Al-composition (x = 0 and 7%), and buffer thickness (from 1.6 to 3.1 μm). A quantitative relationship between the growth conditions and carbon concentration ([C]) is established, which can guide to grow the Ga(Al)N layer with a given [C]. It is found that the carbon incorporation is sensitive to the growth temperature (T) (exponential relationship between [C] and 1/T) and the improvement of breakdown voltage by increasing [C] is observed to be limited when [C] exceeding 1019 cm−3, which is likely due to carbon self-compensation. By increasing the highly resistive (HR) Al0.07Ga0.93N buffer thickness from 1.6 to 3.1 μm, the leakage current is greatly reduced down to 1 μA/mm at a bias voltage of 1000 V.

Stress evolution in AlN and GaN grown on Si(111): experiments and theoretical modeling

We introduce a temperature dependent anisotropic model for the stresses in gallium nitride (GaN) and aluminum nitride (AlN) films grown on Si(111) substrates and their epiwafer bow effects caused by thermal mismatch between the film and substrate. The model is verified by Raman scattering experiments with carefully prepared samples. The stresses analyzed from Raman frequency shifts in experiments show excellent agreement with the stresses from finite element modeling simulations. The interaction force mechanisms and the impact factors are compared. The analysis provides an insight in understanding the defect behaviors in film growth. Our model could be useful in the evaluation of the residual stresses and deformations in film growth control, post thermal process in device manufacture, packaging, and reliability estimation.