Keivan Davami

Thermal conductivity of ZnTe nanowires

The thermal conductivity of individual ZnTe nanowires (NWs) was measured using a suspended micro-bridge device with built-in resistance thermometers. A collection of NWs with different diameters were measured, and strong size-dependent thermal conductivity was observed in these NWs. Compared to bulk ZnTe, NWs with diameters of 280 and 107 nm showed approximately three and ten times reduction in thermal conductivity, respectively. Such a reduction can be attributed to phonon-surface scattering. The contact thermal resistance and the intrinsic thermal conductivities of the nanowires were obtained through a combination of experiments and molecular dynamic simulations. The obtained thermal conductivities agree well with theoretical predictions.

Tuning the mechanical properties of vertical graphene sheets through atomic layer deposition.

We report the fabrication and characterization of graphene nanostructures with mechanical properties that are tuned by conformal deposition of alumina. Vertical graphene (VG) sheets, also called carbon nanowalls (CNWs), were grown on copper foil substrates using a radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) technique and conformally coated with different thicknesses of alumina (Al2O3) using atomic layer deposition (ALD). Nanoindentation was used to characterize the mechanical properties of pristine and alumina-coated VG sheets. Results show a significant increase in the effective Youngs modulus of the VG sheets with increasing thickness of deposited alumina. Deposition of only a 5 nm thick alumina layer on the VG sheets nearly triples the effective Youngs modulus of the VG structures. Both energy absorption and strain recovery were lower in VG sheets coated with alumina than in pure VG sheets (for the same peak force). This may be attributed to the increase in bending stiffness of the VG sheets and the creation of connections between the sheets after ALD deposition. These results demonstrate that the mechanical properties of VG sheets can be tuned over a wide range through conformal atomic layer deposition, facilitating the use of VG sheets in applications where specific mechanical properties are needed.

In situ observation of morphological change in CdTe nano- and submicron wires

We report growth and characterization of CdTe wires 30–400 nm in diameter by the vapor–liquid–solid technique. Individual nanowires were placed on a movable piezotube, which allowed three-dimensional motion toward a scanning tunneling microscope (STM). A bias was applied to the STM tip in contact with the nanowire, and the morphological changes due to Joule heating were observed in situ using a transmission electron microscope (TEM) in real time. For thick CdTe wires (d > ~150 nm), the process results in the growth of superfine nanowires (SFNWs) of 2–4 nm diameter on the surface of the wire. Smaller diameter nanowires, in contrast, disintegrate under the applied bias before the complete evolution of SFNWs on the surface.

Thermal Breakdown of ZnTe Nanowires

As the applications for inorganic nanowires continuously grow, studies on the stability of these structures under high electrical/thermal stress conditions are needed. ZnTe nanowires are grown by the vapor-liquid-solid technique and their breakdown under Joule heating is studied through in situ monitoring in a transmission electron microscope (TEM). The experimental setup, consisting of a scanning tunneling microscope (STM) and a movable piezotube inside the TEM, allows the manipulation of a single nanowire. A voltage applied to the STM tip in contact with a ZnTe nanowire leads to the breakdown of the nanowire into Zn and Te particles or balls which is observed in real time. These balls grow by Ostwald ripening, rendering the surface morphology of the ZnTe nanowire progressively rough. Diffraction patterns along the stem of the wire after the partial breakdown showed substantially smaller lattice spacing compared to 0.35 nm for pristine ZnTe nanowires.

Enhancing the stiffness of vertical graphene sheets through ion beam irradiation and fluorination

Many applications of graphene can benefit from the enhanced mechanical robustness of graphene-based components. We report how the stiffness of vertical graphene (VG) sheets is affected by the introduction of defects and fluorination, both separately and combined. The defects were created using a high-energy ion beam while fluorination was performed in a XeF2 etching system. After ion bombardment alone, the average effective reduced modulus (E r), equal to ∼4.9 MPa for the as-grown VG sheets, approximately doubled to ∼10.0 MPa, while fluorination alone almost quadrupled it to ∼18.4 MPa. The maximum average E r of ∼32.4 MPa was achieved by repeatedly applying fluorination and ion bombardment. This increase can be explained by the formation of covalent bonds between the VG sheets due to ion bombardment, as well as the conversion from sp2 to sp3 and increased corrugation due to fluorination.

Highly sensitive photodetectors using ZnTe/ZnO core/shell nanowire field effect transistors with a tunable core/shell ratio

The fabrication and characterization of a field effect transistor using a radial core/shell structure based on ZnTe nanowires is reported here. The electronic and photoconductive performance of the devices is successfully controlled by tuning the shell to core ratios in the integrated devices. The ZnO shell around the ZnTe nanowire has a significant effect on the optical properties of the transistor, and the photo-to-dark current ratio, responsivity and photoconductive gain are greatly enhanced to 199, 196 and 8.12 × 104% respectively for the 17.5% shell/core ratio. The ability to control the core/shell ratio presented here is promising in device design for optoelectronic applications for covering a wide range of wavelengths.

Nanowires in Thermoelectric Devices

The low efficiency of bulk thermoelectric materials has limited the widespread application of thermoelectric power generation. Theoretical and experimental investigations indicate that materials prepared in the form of nanowires show higher thermoelectric coefficients, thus promising to revolutionize the field. This article reviews the basics of thermoelectric power generation, conventional devices, the role of nanowires and the current status of the field.

Bandgap engineering of Cd(x)Zn(1-x)Te nanowires.

Bandgap engineering of single-crystalline alloy Cd(x)Zn(1-x)Te (0 ≤ x ≤ 1) nanowires is achieved successfully through control of growth temperature and a two zone source system in a vapor-liquid-solid process. Extensive characterization using electron microscopy, Raman spectroscopy and photoluminescence shows highly crystalline alloy nanowires with precise tuning of the bandgap. It is well known that bulk Cd(x)Zn(1-x)Te is popular for construction of radiation detectors and availability of a nanowire form of this material would help to improve detection sensitivity and miniaturization. This is a step forward towards the accomplishment of tunable and predetermined bandgap emissions for various applications.

Modification of mechanical properties of vertical graphene sheets via fluorination

We report systematically tuning the mechanical properties of vertical graphene (VG) sheets through fluorination. VG sheets were synthesized using a radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) technique and were functionalized through exposure to xenon difluoride (XeF2) gas. An atomic force microscopy technique, PeakForce Quantitative Nanomechanical Mapping (QNM®), was used to measure the mechanical properties of the VG sheets. We show that fluorination can significantly enhance the reduced modulus of surfaces comprised of VG sheets. Samples with only ∼3.5% fluorine had a reduced modulus approximately eight times higher than unfunctionalized VG sheets, which is attributed to sp2 to sp3 conversion and a change of the C–C bond length after functionalization. Fluorination also decreased the energy dissipation of the VG sheets and reduced their adhesion to the AFM tip. This method represents a unique approach towards modification of the mechanical properties of nanostructures without a significant increase in weight or change of the VG sheet morphology.

A Study on the Stoichiometry of One-Dimensional Nanostructures

While attributes such as small dimensions, low power consumption, fast sensor response, and a wide range of detection give one-dimensional nanostructures excellent potential to revolutionize sensor and detector industries, challenges to achieving uniform stoichiometry pose significant obstacles to their commercial use. Diverse characteristics arise from nanostructures with variable compositions and morphologies. Thus, investigation of physical properties of nanostructures would be pointless if one cannot assure the exact stoichiometry of the material. We studied the stoichiometry of ZnTe nanowires grown via the vapor-liquid-solid method. Different microscopy and composition analysis methods were exploited to study the stoichiometry of the nanowires. It was observed that nonstoichiometric wires had relatively higher defect concentrations. The temperature profile along the substrate during nanowire growth was found to be the reason for the formation of nanowires with different stoichiometries.