Hongqi Xu

InP Nanowire Array Solar Cells Achieving 13.8% Efficiency by Exceeding the Ray Optics Limit

Improving Nanowire Photovoltaics In principle, solar cells based on arrays of nanowires made from compound inorganic semiconductors, such as indium phosphide (InP), should decrease materials and fabrication costs compared with planar junctions. In practice, device efficiencies tend to be low because of poor light absorption and increased rates of unproductive charge recombination in the surface region. Wallentin et al. (p. 1057, published online 17 January) now report that arrays of p-i-n InP nanowires (that switch from positive to negative doping), grown to millimeter lengths, can be optimized by varying the nanowire diameter and length of the n-doped segment. Efficiencies as high as 13.8% were achieved, which are comparable to the best planar InP photovoltaics. Nanowire solar cells were fabricated that exhibit high photocurrents and low surface recombination. Photovoltaics based on nanowire arrays could reduce cost and materials consumption compared with planar devices but have exhibited low efficiency of light absorption and carrier collection. We fabricated a variety of millimeter-sized arrays of p-type/intrinsic/n-type (p-i-n) doped InP nanowires and found that the nanowire diameter and the length of the top n-segment were critical for cell performance. Efficiencies up to 13.8% (comparable to the record planar InP cell) were achieved by using resonant light trapping in 180-nanometer-diameter nanowires that only covered 12% of the surface. The share of sunlight converted into photocurrent (71%) was six times the limit in a simple ray optics description. Furthermore, the highest open-circuit voltage of 0.906 volt exceeds that of its planar counterpart, despite about 30 times higher surface-to-volume ratio of the nanowire cell.

Anomalous Zero-Bias Conductance Peak in a Nb-InSb Nanowire-Nb Hybrid Device.

M. T. Deng,1 C. L. Yu,1 G. Y. Huang, 1 M. Larsson, 1 P. Caroff,2 and H. Q. Xu1, 3,∗ Division of Solid State Physics, Lund University, Box 118, S -221 00 Lund, Sweden 2I.E.M.N., UMR CNRS 8520, Avenue Poincaré, BP 60069, F-5965 2 Villeneuve d’Ascq, France 3Department of Electronics and Key Laboratory for the Physic s and Chemistry of Nanodevices, Peking University, Beijing 100871, China (Dated: March 27, 2012)Semiconductor InSb nanowires are expected to provide an excellent material platform for the study of Majorana fermions in solid state systems. Here, we report on the realization of a Nb-InSb nanowire-Nb hybrid quantum device and the observation of a zero-bias conductance peak structure in the device. An InSb nanowire quantum dot is formed in the device between the two Nb contacts. Due to the proximity effect, the InSb nanowire segments covered by the superconductor Nb contacts turn to superconductors with a superconducting energy gap Δ(InSb) ∼ 0.25 meV. A tunable critical supercurrent is observed in the device in high back gate voltage regions in which the Fermi level in the InSb nanowire is located above the tunneling barriers of the quantum dot and the device is open to conduction. When a perpendicular magnetic field is applied to the devices, the critical supercurrent is seen to decrease as the magnetic field increases. However, at sufficiently low back gate voltages, the device shows the quasi-particle Coulomb blockade characteristics and the supercurrent is strongly suppressed even at zero magnetic field. This transport characteristic changes when a perpendicular magnetic field stronger than a critical value, at which the Zeeman energy in the InSb nanowire is E(z) ∼ Δ(InSb), is applied to the device. In this case, the transport measurements show a conductance peak at the zero bias voltage and the entire InSb nanowire in the device behaves as in a topological superconductor phase. We also show that this zero-bias conductance peak structure can persist over a large range of applied magnetic fields and could be interpreted as a transport signature of Majorana fermions in the InSb nanowire.

Covalently bonded single-molecule junctions with stable and reversible photoswitched conductivity

Stable molecular switches Many single-molecule current switches have been reported, but most show poor stability because of weak contacts to metal electrodes. Jia et al. covalently bonded a diarylethene molecule to graphene electrodes and achieved stable photoswitching at room temperature (see the Perspective by Frisbie). The incorporation of short bridging alkyl chains between the molecule and graphene decoupled their pielectron systems and allowed fast conversion of the open and closed ring states. Science, this issue p. 1443; see also p. 1394 Stable molecular conduction junctions were formed by covalently bonding single diarylethenes to graphene electrodes. Through molecular engineering, single diarylethenes were covalently sandwiched between graphene electrodes to form stable molecular conduction junctions. Our experimental and theoretical studies of these junctions consistently show and interpret reversible conductance photoswitching at room temperature and stochastic switching between different conductive states at low temperature at a single-molecule level. We demonstrate a fully reversible, two-mode, single-molecule electrical switch with unprecedented levels of accuracy (on/off ratio of ~100), stability (over a year), and reproducibility (46 devices with more than 100 cycles for photoswitching and ~105 to 106 cycles for stochastic switching).

Electrical properties of three-terminal ballistic junctions

Based on the ballistic nature of electron transport, exploitable nonlinear transport phenomena are predicted for three-terminal ballistic junctions (TBJs). For a symmetric TBJ, it is shown that when finite voltages Vl and Vr are applied in push-pull fashion, with Vl=V and Vr=−V, to the left and right branches, the voltage output Vc from the central branch will always be negative. This characteristic appears even when the device symmetry is broken, provided that |V| is greater than a certain threshold. It is also shown that the TBJs exhibit parabolic behavior for Vc vs V, in the weak nonlinear response regime. Potential applications of these devices in nanoelectronics are discussed.

Unified Treatment of Fluorescence and Raman Scattering Processes near Metal Surfaces

We present a general model study of surface-enhanced resonant Raman scattering and fluorescence focusing on the interplay between electromagnetic effects and the molecular dynamics. Our model molecule is placed close to two Ag nanoparticles and has two electronic levels. A Franck-Condon mechanism provides electron-vibration coupling. Using realistic parameter values for the molecule we find that an electromagnetic enhancement by 10 orders of magnitude can yield Raman cross sections sigma(R) of the order 10(-14) cm(2). We also discuss the dependence of sigma(R) on incident laser intensity.

Nonlinear operation of GaInAs/InP-based three-terminal ballistic junctions

We report on nonlinear electrical properties of three-terminal ballistic junctions (TBJs) based on high-electron-mobility GaInAs/InP quantum-well structures. Nonlinear electrical transport behavior of the TBJs is found, and we show a correlation between this behavior and the linear regime of electron transmission in the devices. We also study device geometry effects on these electrical properties of the TBJs. Finally, we demonstrate room-temperature operation of the devices. The results obtained are compared with recent predictions by Xu [H. Q. Xu, Appl. Phys. Lett. 78, 2064 (2001)] and good agreement is found.

Giant, Level-Dependent g Factors in InSb Nanowire Quantum Dots

We report on magnetotransport measurements on InSb nanowire quantum dots. The measurements show that the quantum levels of the InSb quantum dots have giant g factors, with absolute values up to approximately 70, the largest value ever reported for semiconductor quantum dots. We also observe that the values of these g factors are quantum level dependent and can differ strongly between different quantum levels. The presence of giant g factors indicates that considerable contributions from the orbital motion of electrons are preserved in the measured InSb nanowire quantum dots, while the level-to-level fluctuations arise from spin-orbit interaction. We have deduced a value of Delta(SO) = 280 mueV for the strength of spin-orbit interaction from an avoided level crossing between the ground state and first excited state of an InSb nanowire quantum dot with a fixed number of electrons.

Thermoelectric efficiency at maximum power in low-dimensional systems

Low-dimensional electronic systems in thermoelectrics have the potential to achieve high thermal-to-electric energy conversion efficiency. A key measure of performance is the efficiency when the device is operated under maximum power conditions. Here we study the efficiency at maximum power, in the absence of phonon-mediated heat flow, of three low-dimensional, thermoelectric systems: a zero-dimensional quantum dot with a Lorentzian transmission resonance of finite width, a one-dimensional (1D) ballistic conductor, and a thermionic (TI) power generator formed by a two-dimensional energy barrier. In all three systems, the efficiency at maximum power is independent of temperature, and in each case a careful tuning of relevant energies is required to achieve maximal performance. We find that quantum dots perform relatively poorly under maximum power conditions, with relatively low efficiency and small power throughput. Ideal one-dimensional conductors offer the highest efficiency at maximum power (36% of the Carnot efficiency). Whether 1D or TI systems achieve the larger maximum power output depends on temperature and area filling factor. These results are also discussed in the context of the traditional figure of merit ZT. (Less)

Bias-voltage-induced asymmetry in nanoelectronic Y-branches

Pronounced asymmetries of electrical properties are observed in nanoelectronic, symmetric GaAs/AlGaAs Y-branches. Finite voltages Vl and Vr applied to the left- and right-hand side branch reservoir of a symmetric, ballistic Y-branch switching device in push–pull fashion (i.e., Vl=−Vr) lead to a negative output voltage Vs of the floating, central stem reservoir located between the two branches. We explain our observations exploiting the ballistic nature of the electron transport in the device.

Photovoltaics with Piezoelectric Core—Shell Nanowires

We report on a theoretical discovery of a generic piezoelectric field in strained core-shell compound semiconductor nanowires. We show, using both an analytical model and numerical simulations based on fully electroelastically coupled continuum elasticity theory, that lattice-mismatch-induced strain in an epitaxial core-shell nanowire gives rise to an internal electric field along the axis of the nanowire. This piezoelectric field results predominantly from atomic layer displacements along the nanowire axis within both the core and shell materials and can appear in both zinc blende and wurtzite crystalline core-shell nanowires. The effect can be employed to separate photon-generated electron-hole pairs in the core-shell nanowires and thus offers a new device concept for solar energy conversion.