In-place measurement of semiconductor nanowires can be achieved using a nanoprobe inside a scanning electron microscope. Gold catalyzed vapour-liquid-solid nanowires exhibit a highly perfect metal semiconductor interface which is generally rectifying for moderate to low doping values. The electrical properties of the semiconducting region can be inferred from careful attention to the IV properties in a rapid measurement process involving no lithography or sidewall degradation. The technique is useful for evaluating the effect of surface states on the nanowire conductivity. We present initial results on the epitaxy and electrical characterization of core-shell p-n junction structures using the nanoprobe method.

Commercially available light-emitting diodes (LEDs) suffer from low-efficiency in the green region of the visible spectrum. In order to solve this issue III-V materials such as Gallium phosphide (GaP) can be investigated. GaP in the zinc blende (ZB) crystal structure has an indirect band gap, limiting the efficiency of the green emission. However, when the material is grown with wurtzite (WZ) crystal phase a direct band gap is predicted. Here, we show the fabrication and the characterization of wurtzite GaP nanowires, together with the demonstration of the direct band gap. The strong photoluminescence signal observed at 594 nm with a lifetime in the order of 1ns matches with the expectation for a direct band gap material. Furthermore, the emission wavelength can be tuned across a wide range of the visible spectrum (555−690 nm) by incorporating aluminum or arsenic in the WZ GaP nanowires.

Nanostructures provide novel opportunities of studying epitaxy in nano/mesoscale and on nonplanar substrates. Epitaxial growth of silicon (Si) on the surfaces of Si nanowires along radial direction is a promising way to prepare radial p-(i)-n junction in nanoscale for optoelectronic devices. Comprehensive studies of Si radial epitaxy in micro/nanoscale reveal that morphological evolution and size-dependent radial shell growth rate for undoped and doped Si radial shells. Single crystalline Si radial p-i-n junction wire arrays were utilized to fabricate photovoltaic (PV) devices. The PV devices exhibited the photoconversion efficiency of 10%, the short-circuit current density of 39 mA/cm², and the open-circuit voltage of 0.52 V, respectively.

The use of GaN crystals grown by three methods (and their combinations): Hydride Vapor Phase Epitaxy (HVPE), high nitrogen pressure solution (HNPS) and ammonothermal method for optoelectronic (laser diodes) and electronic (transistors) devices is presented. After a brief review on the development of the three crystallization methods, the GaN crystals’ uniform and unique properties, which allow to use them as substrates for building devices, are shown. The Metal Organic Vapor Phase Epitaxy (MOCVD) and Molecular Beam Epitaxy (MBE) technologies for growing the nitride quantum nanostructures as well as the structures’ properties and processing of devices are demonstrated. Future challenges and perspectives for application of bulk GaN as substrates in building quantum nanostructures for some electronic and optoelectronic devices are discussed.

The growth of thin Bi₂Se₃ films on (0001) sapphire substrates by metalorganic chemical vapor deposition (MOCVD) was investigated. A two-heater configuration was employed to pre-crack the metalorganic sources upstream of the substrate while maintaining a low substrate temperature (<250°C). Epitaxial Bi₂Se₃ films with (006) x-ray rocking curve full-width-at-half-maximum values on the order of 160 arcsecs were obtained at growth rates of ~6 nm/min or lower while higher growth rates resulted in polycrystalline films. The background electron concentration of the films was found to depend strongly on the substrate temperature and Se/Bi inlet ratio. Bi₂Se₃ films with a room temperature electron concentration of 6.7x10¹⁹ cm^-3 and mobility of 155 cm²/Vs were obtained at 200°C with a Se/Bi ratio of 80. Higher substrate temperature and lower Se/Bi ratios resulted in an increase in electron concentration and corresponding reduction in mobility. The results demonstrate the potential of MOCVD for the growth of Bi₂Se₃ and related materials for topological insulator studies.

The magnetic doping of Ge-QDs is highly coveted and has been pursued for several years. The Ge- Mn-Si substrate system presents a complex challenge, and the competition between dopant integration and formation of silicides, germanides and metastable phases makes the magnetic doping a considerable, and maybe insurmountable challenge. We will discuss the interaction of Mn with all growth surface, Si(100), Ge(100) wetting layer, and Ge{105} QD facet, as well as the co-deposition of Mn and Ge. The monoatomic Mn-wires, which form on Si(100), and their magnetic signatures allow unique insight into the relation between bonding and magnetism. We will close with an outlook on the feasibility of QD manipulation by controlling dopant-surface interactions.

We report on the effects of changing the surface densities of MOVPE-grown free-standing GaAs-AlGaAs core-shell nanowires on the resulting nanostructure size and their photoluminescence (PL) properties. It is demonstrated that decreasing the local density of GaAs nanowires within the array leads to an increase of the overgrown AlGaAs shell thickness and to a substantial redshift of the nanostructure excitonic emission. Application of a vapor mass-transport limited growth model of the AlGaAs shell allows explaining the dependence of shell growth rate on nanowire density. The observed redshift of the nanowire PL emission is then experimentally correlated with these density-induced changes of the nanostructure size, namely with the nanowire shell-thickness to core-radius ratio hs/Rc. To account for a possible contribution of the nanostructure built-in elastic strain to the energy shift of the peak excitonic emission, the strain field in present core-shell nanowires was calculated as function of the nanostructure relevant geometrical parameters, based on a uniaxial elastic energy equilibrium model, and its effect on valence and conduction band shifts of the GaAs core evaluated by means of the Pikus-Bir Hamiltonian. Good agreement is obtained for hs/Rc<1, the strain-free excitonic emission being identified at 1.510 eV and ascribed to bound heavy-hole excitons. For hs/Rc>1 increasingly larger redshifts (up to ~9 meV in excess of values calculated based on the elastic strain model) are observed, and tentatively ascribed to shell-dependent exciton localization effects.

To realize extreme-scale neuromorphic computation inspired by a biological brain, there is a need to develop two-terminal reconfigurable devices that can mimic the low-power specifications and scalability of a biological synapse. This paper discusses the synaptic characteristics of doped transition metal oxide based two-terminal devices. Spike-frequency dependent augmentation in conductance was observed. In addition, the devices could be reconfigured to different conductance states by changing the input pulse-width. This characteristic was used to demonstrate spike-timing dependent plasticity (STDP). The mechanism of reconfiguration is also briefly discussed.

Recently resistive switching (RS) based on ZnO thin film has attracted considerable attention since ZnO with doping can improve the switching ratio and device performance. In this work, Cu/ZnO/AZO (Al-doped ZnO) and Cu/ZnO:Cu (Cudoped ZnO) /AZO structures were fabricated for RS, using AZO as bottom electrodes due to its lattice matching with ZnO, and metal Cu was deposited as the top electrodes. The current-voltage (I-V) characteristics of these RS devices using different doped ZnO thin films as a dielectric layer were analyzed and compared. The results demonstrated that ZnO:Cu RS had a higher switching ratio and a larger range of setup and reset voltage than ZnO RS. In addition, we also found that the high resistance state（HRS）and the low resistance state (LRS) were accordance with space charge limited current (SCLC) and Ohm’s law respectively. In addition, the effect on RS performance by the top electrode was investigated by depositing top electrode with different sizes and annealing treatment, and the results indicate that the RS phenomenon occurred in these Cu/ZnO:Cu/AZO structure devices is caused by bulk effect and interfacial effect synthetically.

We present synthesis, acid-leaching, characterization and electrochemistry of α-MnO₂ nanowires with tunnel crystal structure. This material is used as a matrix for lithium ions intercalation to provide insights into the effects of postsynthesis treatment on charge storage properties. Hydrothermal treatment of precursors produced 20 - 200 nm thick and tens of microns long nanowires. Acid leaching was carried out in the concentrated nitric acid at room temperature and resulted in the change of material composition and surface area. Original α-MnO₂ nanowires showed initial discharge specific capacity of 96 mAh/g, while acid-leached material exhibited higher capacity values. This work forms the basis for future study aimed at understanding of correlation between crystal structure, composition and morphology of the “host” matrix and nature of the “guest” ions for beyond lithium electrochemical energy storage. In addition, we demonstrate single nanowire electrochemical cells for the study of electrochemically-correlated mechanical properties of the nanowires.

The glancing angle deposition (GLAD) technique, unlike a conventional physical vapor deposition (PVD) process, incorporates a flux of atoms that are obliquely incident on a tilted and rotating substrate. Instead of a continuous thin film coating, these atoms can form arrays of three-dimensional nanostructures due to a shadowing effect. By simply controlling the deposition angle and substrate rotation speed, nanostructures of a large variety of materials in the shapes of rods, screws, or springs can be obtained easily that are otherwise difficult to produce by conventional lithographical techniques. In this study, a brief overview of the growth mechanisms of GLAD nanostructures is presented. In addition, a new small angle deposition (SAD) technique as a simple means of conformally coating nanorod or nanowire arrays is described. SAD utilizes a small tilt angle during PVD on nanostructured substrates, which allows the effective exposure of nanorod sidewalls to the incoming flux and leads to enhanced thin film conformality. In this work, some recent results on core-shell nanorod arrays obtained by coating GLAD nanorods with a SAD shell will be presented. It will be shown that core-shell nanostructured geometries obtained by the simple SAD-GLAD method can significantly enhance catalyst activity for fuel cell electrodes, and charge carrier collection efficiency in photoconductive/semiconductor nanostructured materials.

It is commonly believed that small nanowires make more sensitive bioFETs than larger nanowires and planar structures because of the high surface-area-to-volume ratio that is the result of width scaling. We believe this justification is incorrect because it ignores the effect of varying radius of curvature on Debye screening. We suggest an alternative explanation for the enhancements seen in nanowire bioFETs. Our results suggest that properly engineered large-scale structures can achieve sensitivities on par with nanowires.