Phonons—quanta of crystal lattice vibrations—reveal themselves in electrical, thermal, optical, and mechanical phenomena in materials. Phonons carry heat, scatter electrons, and affect light–matter interactions. Nanostructures opened opportunities for tuning the phonon spectrum and related properties of materials for specific applications, thus realizing what was termed phonon engineering. Recent progress in graphene and two-dimensional van der Waals materials has led to a better understanding of phonon physics and created additional opportunities for controlling phonon interactions and phonon transport at room temperature. This article reviews the basics of phonon confinement effects in nanostructures, describes phonon thermal transport in graphene, discusses phonon properties of van der Waals materials, and outlines practical applications of low-dimensional materials that rely on phonon properties.

Graphene reveals many extraordinary properties including extremely high room temperature carrier mobility and intrinsic thermal conductivity. Understanding how to controllably modify graphene’s properties is essential for its proposed applications. Here we report on a method for tuning the electrical properties of graphene via electron beam irradiation. It was observed that single-layer graphene is highly susceptible to the low-energy electron beams. We demonstrated that by controlling the irradiation dose one can change, by desired amount, the carrier mobility, shift the charge neutrality point, increase the resistance at the minimum conduction point, induce the “transport gap” and achieve current saturation in graphene. The change in graphene properties is due to defect formation on the graphene surface and in the graphene lattice. The changes are reversible by annealing until some critical irradiation dose is reached.

A large-area graphene layer identification technique was developed for research and industrial applications. It is based on the analysis of optical microscopy images using computational image processing algorithms. The initial calibration is performed with the micro-Raman spectroscopy. The method can be applied to the wafer-scale graphene samples. The technique has the potential to be the gateway in the development of fully automated statistical process control methods for the next generation thin-film materials used by the semiconductor industry. The proposed technique can be applied to graphene on arbitrary substrates and used for other atomically thin materials.

We used a “graphene-like” mechanical exfoliation to obtain atomically thin films of TiTe2. The building blocks of titanium ditelluride are atomic tri-layers separated by the van der Waals gaps. The exfoliation procedure allows one to obtain the few-atom-thick films with strong confinement of charge carriers and phonons. We have verified the crystallinity of the exfoliated films and fabricated the back-gated field-effect devices. The current – voltage characteristics of the TiTe2 devices revealed strong non-linearity, which suggests the charge-density wave effects. The obtained results are important for the proposed application of TiTe2 for the charge-density wave devices and thermoelectric energy conversion.

In this letter, single stranded Deoxyribonucleic Acids (ssDNA) are found to act as negative potential gating agents that increase the hole density in single layer graphene (SLG). Current-voltage measurement of the hybrid ssDNA/graphene system indicates a shift in the Dirac point and “intrinsic” conductance after ssDNA is patterned. The effect of ssDNA is to increase the hole density in the graphene layer, which is calculated to be on the order of 1.8×1012 cm-2. This increased density is consistent with the Raman frequency shifts in the G-peak and 2D band positions and the corresponding changes in the G-peak full-width half maximum. This patterning of DNA on graphene layers could provide new avenues to modulate their electrical properties and for novel electronic devices.

In recent years, there has been an increasing interest in thermal properties of materials. This arises mostly from the practical needs of heat removal and thermal management, which have now become critical issues for the continuing progress in electronic and optoelectronic industries. Another motivation for the study of thermal properties at nanoscale is from a fundamental science perspective. Thermal conductivity of different allotropes of carbon materials span a uniquely large range of values with the highest in graphene and carbon nanotube and the lowest in amorphous or disordered carbon. Here we describe the thermal properties of graphene and carbon-based materials and analyze the prospects of applications of carbon materials in thermal management.

Graphene is a promising candidate material for thermal management of high-power electronics owing to its high intrinsic thermal conductivity. Here we report preliminary results of the proof-of-concept demonstration of graphene lateral heat spreaders. Graphene flakes were transferred on top of GaN devices through the mechanical exfoliation method. The temperature rise in the GaN device channels was monitored in-situ using micro-Raman spectroscopy. The local temperature was measured from the shift in the Raman peak positions. By comparing Raman spectra of GaN devices with and without graphene heat spreader, we demonstrated that graphene lateral heat spreaders effectively reduced the local temperature by ~ 20oC for a given dissipated power density. Numerical simulation of heat dissipation in the considered device structures gave results consistent with the experimental data.

We carried out a systematic experimental study of the low-frequency noise characteristics in a large number of single and bilayer graphene transistors. The prime purpose was to determine the dominant noise sources in these devices and the effect of aging on the current-voltage and noise characteristics. The analysis of the noise spectral density dependence on the surface area of the graphene channel indicates that the dominant contributions to the 1/f electronic noise come from the graphene channel region itself. Aging of graphene transistors due to exposure to ambient for over a month resulted in substantially increased noise, which was attributed to the decreasing mobility of graphene and increasing contact resistance. The noise spectral density in both single and bilayer graphene transistors shows a non-monotonic dependence on the gate bias. This observation confirms that the 1/f noise characteristics of graphene transistors are qualitatively different from those of conventional silicon metal-oxide-semiconductor field-effect transistors.

We fabricated a number of top-gate graphene field-effect transistors on the ultrananocrystalline diamond (UNCD) – Si composite substrates. Raman spectroscopy, scanning electron microscopy and atomic force microscopy were used to verify the quality of UNCD and graphene device channels. The thermal measurements were carried out with the “hot disk” and “laser flash” methods. It was found that graphene on UNCD devices have increased breakdown current density by ∼50% compared to the reference devices fabricated on Si/SiO2. The relatively smooth surface of UNCD, as compared to other synthetic diamond films, allowed us to fabricate top gate graphene devices with the drift mobility of up to ∼ 2587 cm2V-1s-1.

We theoretically studied how the electric filed effect can modify thermoelectric properties of intrinsic bismuth telluride nanowires, which are grown along [110] direction. The electronic structure and wave functions were calculated by solving the self-consistent system of the Schrodinger and Poisson equations by means of both the Thomas-Fermi approximation and the spectral element method. The thermoelectric parameters were determined using a constant relaxation-time approximation. The external electric field can increase the Seebeck coefficient of a nanowire with 7 - 15 nm lateral dimensions by nearly a factor of two, and enhance the figure of merit by an order of magnitude.

Two-dimensional electro-thermal simulations of GaN-based metal-semiconductor field-effect transistor are performed in the framework of the drift-diffusion model. The dependence of the hot spot temperature in transistors with many gates on the gate-to-gate pitch is studied. The case of SiC substrate is compared to the case of sapphire substrate. The ambient temperature effect on transistor performance is simulated. The specific of a thermal breakdown in GaN-based devices is discussed. The results obtained can be useful for the optimization of the thermal design for field-effect transistors.

Nanostructures made of zinc oxide (ZnO), a wide-bandgap semiconductor, have recently attracted attention due to their proposed applications in low-voltage and short-wavelength (368 nm) electro-optical devices, transparent ultraviolet (UV) protection films, gas sensors, and varistors. Raman spectroscopy presents a powerful tool for identifying specific materials in complex structures and for extracting useful information on properties of nanoscale objects. At the same time the origin of Raman peak deviation from the bulk values is not always well understood for new material systems. There are three main mechanisms that can induce phonon shifts in the free-standing undoped ZnO nanostructures: (i) phonon confinement by the quantum dot boundaries; (ii) phonon localization on defects and (iii) the laser-induced heating in nanostructure ensembles. Here, we report results of the combined non-resonant and resonant Raman scattering studies of an ensemble of ZnO quantum dots with diameter 20 nm. Based on our experimental data, we have been able to identify the origin of the observed phonon frequency shifts. It has been found that the ultraviolet laser heating of the ensemble induces a large red shift of the phonon frequencies. It is calculated that the observed red shift of 14 cm-1 corresponds to the local temperature of the quantum dot ensemble of about 700°C.

Properly designed sequences of oligonucleotides can be employed as scaffolds or templates for the self-organization of nanostructures and devices, through the Watson-Crick base pairing mechanism which serves as a programmable smart glue. In this paper, we report the Platinum metallization of peptide nucleic acid (PNA) sequences for the first time. PNA is an analogue of DNA and has a neutral backbone which provides stronger hybridization, greater stability and higher specificity in base pairing. Pt ions were reduced from a salt solution and localized over the PNA fragments where the size of the Pt colloids depends on the duration of chemical reduction. Computations of the high lying occupied and lowlying unoccupied orbitals indicated that Pt nanoparticles bind easily on both the Thymine (T) bases and the backbone in the PNA.

We have investigated exciton states in wurtzite GaN/AlN and ZnO quantum dots. A strong piezoelectric field in GaN/AlN quantum dots is found to tilt conduction and valence bands, thus pushing the electron to the top and the hole to the bottom of the GaN/AlN quantum dot. As a result, the exciton ground state energy in GaN/AlN quantum dots with heights larger than 3 nm exhibits a red shift with respect to bulk GaN energy gap. It is shown that the radiative decay time in GaN/AlN quantum dots is large and increases from 0.3 ns for quantum dots with height 1.5 nm to 1.1×103 ns for the quantum dots with height 4.5 nm. On the contrary, the electron and the hole are not separated in ZnO quantum dots. Moreover, a relatively thick “dead layer” is formed near the surface of ZnO quantum dots. As a result, the radiative decay time in ZnO quantum dots is small and decreases from 73 ps for quantum dots with diameter 1.5 nm to 29 ps for the quantum dots with diameter 6 nm.

We theoretically investigate the thermal boundary resistance and heat diffusion in AlGaN/GaN heterostructure field-effect transistors. Our calculations based on the diffuse mismatch model show that the thermal boundary resistance at the interface between GaN and SiC can strongly influence the temperature rise in the device channel.

We apply Klemens' second-order perturbation theory to investigate thermal conductivity in wurtzite GaN films. Specifically, the effect of edge, screw, and mixed types of dislocations and their orientation with respect to the temperature gradient on the thermal conductivity values is analyzed. Using typical impurity profiles for GaN films, we study the relative contribution of different impurities into thermal resistance.

We describe a model for numerical calculation of phonon spectrum in a three-dimensional regimented array of semiconductor quantum dots. Regimentation and possibility of carrier mini-band formation make this structure analogous to a crystal, e.g. quantum dot crystal. It is demonstrated that the acoustic phonon dispersion undergoes strong modification in such a structure leading to emergence of low-energy quasi-optical branches. Strong phonon spectrum modification is expected to affect carrier relaxation and transport properties.

The interface trap density NT is extracted from the experimental low-frequency noise data for GaN/AlGaN heterostructure field-effect transistors (HFETs). The trap density is determined based on the carrier-density fluctuation formalism. We show that the value of NT approximately defines the noise response of different GaN/AlGaN HFETs fabricated on the same wafer and it weakly depends on the gate bias. The dependence is due to the non-uniformity of the trap distribution. A model for computer simulation of the low-frequency noise in GaN devices is proposed.

We analyze the electron energy spectrum in three-dimensional regimented arrays of semiconductor quantum dots. The coupling among quantum dots results in formation of three- dimensional electron mini-bands. Changing the size of quantum dots, inter-dot distance, barrier height and regimentation, one can control the electronic band structure of this quantum dot superlattice, which can also be referred to as quantum dot crystal due to its structure and energy spectrum that resemble those of a real crystal. Results of computer simulations carried out for a tetragonal InAs/GaAs quantum dot superlattice show that the electron density of states, effective mass tensor and other properties are different from those of bulk and conventional quantum well superlattices.