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We explore the effect of charge density wave (CDW) on the in-plane thermoelectric transport properties of (PbSe)1+δ(VSe2)1 and (PbSe)1+δ(VSe2)2 heterostructures. In (PbSe)1+δ(VSe2)1 we observe an abrupt 86% increase in the Seebeck coefficient, 245% increase in the power factor, and a slight decrease in resistivity over the CDW transition. This behavior is not observed in (PbSe)1+δ(VSe2)2 and is rather unusual compared to the general trend observed in other materials. The abrupt transition causes a deviation from the Mott relationship through correlated electron states. Raman spectra of the (PbSe)1+δ(VSe2)1 material show the emergence of additional peaks below the CDW transition temperature associated with VSe2 material. Temperature-dependent in-plane X-ray diffraction (XRD) spectra show a change in the in-plane thermal expansion of VSe2 in (PbSe)1+δ(VSe2)1 due to lattice distortion. The increase in the power factor and decrease in the resistivity due to CDW suggest a potential mechanism for enhancing the thermoelectric performance at the low temperature region.

We demonstrate the hot electron injection of photoexcited carriers in an Ag-based plasmon resonant grating structure. By varying the incident angle of irradiation, sharp dips are observed in the reflectance with p-polarized light (electric field perpendicular to grating lines) when there is wavevector matching between the incident light and the plasmon resonant modes of the grating and no angle dependence is observed with s-polarized light. This configuration enables us to compare photoelectrochemical current produced by plasmon resonant excitation with that of bulk metal interband absorption simply by rotating the polarization of the incident light while keeping all other parameters of the measurement fixed. With 633 nm light, we observed a 12-fold enhancement in the photocurrent (i.e., reaction rate) between resonant and nonresonant polarizations at incident angles of ±7.6° from normal. At 785 nm irradiation, we observed similar resonant profiles to those obtained with 633 nm wavelength light but with a 44-fold enhancement factor. Using 532 nm light, we observed two resonant peaks (with approximately 10× enhancement) in the photocurrent at 19.4° and 28.0° incident angles, each corresponding to higher order modes in the grating with more nodes per period. The lower enhancement factors observed at shorter wavelengths are attributed to interband transitions, which provide a damping mechanism for the plasmon resonance. Finite difference time domain (FDTD) simulations of these grating structures confirm the resonant profiles observed in the angle-dependent spectra of these gratings and provide a detailed picture of the electric field profiles on and off resonance.

We report a comparison of the photocatalytic performance of WSe2-on-MoSe2 and MoSe2-on-WSe2 heterostructures. While built-in electric fields exist in these heterostructures on the order of 100 kV/c...

Understanding the relaxation and injection dynamics of hot electrons is crucial to utilizing them in photocatalytic applications. While most studies have focused on hot carrier dynamics at metal/semiconductor interfaces, we study the in situ dynamics of direct hot electron injection from metal to adsorbates. Here, we report a hot electron-driven hydrogen evolution reaction (HER) by exciting the localized surface plasmon resonance (LSPR) in Au grating photoelectrodes. In situ ultrafast transient absorption (TA) measurements show a depletion peak resulting from hot electrons. When the sample is immersed in solution under -1 V applied potential, the extracted electron-phonon interaction time decreases from 0.94 to 0.67 ps because of additional energy dissipation channels. The LSPR TA signal is redshifted with delay time because of charge transfer and subsequent change in the dielectric constant of nearby solution. Plateau-like photocurrent peaks appear when exciting a 266 nm linewidth grating with p-polarized (on resonance) light, accompanied by a similar profile in the measured absorptance. Double peaks in the photocurrent measurement are observed when irradiating a 300 nm linewidth grating. The enhancement factor (i.e., reaction rate) is 15.6× between p-polarized and s-polarized light for the 300 nm linewidth grating and 4.4× for the 266 nm linewidth grating. Finite-difference time domain (FDTD) simulations show two resonant modes for both grating structures, corresponding to dipolar LSPR modes at the metal/fused silica and metal/water interfaces. To our knowledge, this is the first work in which LSPR-induced hot electron-driven photochemistry and in situ photoexcited carrier dynamics are studied on the same plasmon resonance structure with and without adsorbates.

We report plasmonic enhancement of photocatalysis by depositing 5 nm Au nanoislands onto tungsten diselenide (WSe2) monolayer films. Under 532 nm wavelength illumination, the bare WSe2 film produce...

It has been known for several decades that defects are largely responsible for the catalytically active sites on metal and semiconductor surfaces. However, it is difficult to directly probe these active sites because the defects associated with them are often relatively rare with respect to the stoichiometric crystalline surface. In the work presented here, we demonstrate a method to selectively probe defect-mediated photocatalysis through differential alternating current (ac) photocurrent (PC) measurements. In this approach, electrons are photoexcited from the valence band to a relatively narrow distribution of subband gap states in TiO2 and then subsequently to the ions in solution. Because of their limited number, these defect states fill up quickly, resulting in Pauli blocking, and are thereby undetectable under direct current or continuous wave excitation. In the method demonstrated here, the incident light is modulated with an optical chopper, whereas the PC is measured with a lock-in amplifier. Thin (5 nm) films of TiO2 deposited by atomic layer deposition on various metal films, including Au, Cu, and Al, exhibit the same wavelength-dependent PC spectra, with a broad peak centered around 2.0 eV corresponding to the band-to-defect transition associated with the hydrogen evolution reaction (HER). While the UV-vis absorption spectra of these films show no features at 2.0 eV, photoluminescence (PL) spectra of these photoelectrodes show a similar wavelength dependence with a peak of around 2.0 eV, corresponding to the subband gap emission associated with these defect sites. As a control, alumina (Al2O3) films exhibit no PL or PC over the visible wavelength range. The ac PC plotted as a function of electrode potential shows a peak of around -0.4 to -0.1 V versus normal hydrogen electrode, as the monoenergetic defect states are tuned through a resonance with the HER potential. This approach enables the direct photoexcitation of catalytically active defect sites to be studied selectively without the interference of the continuum interband transitions or the effects of Pauli blocking, which is limited by the slow turnover time of the catalytically active sites, typically on the order of 1 μs. We believe that this general approach provides an important new way to study the role of defects in catalysis in an area where selective spectroscopic studies of these are few.

Plasmon resonant grating structures provide an effective platform for distinguishing between the effects of plasmon resonant excitation and bulk metal absorption via interband transitions. By simply rotating the polarization of the incident light, we can switch between resonant excitation and non-resonant excitation, while keeping all other parameters of the measurement constant. With light polarized perpendicular to the lines in the grating (i.e., TE-polarization), the photocatalytic reaction rate (i.e., photocurrent) is measured as the angle of the incident laser light is tuned through the resonance with the grating. Here, hot holes photoexcited in the metal are used to drive the oxygen evolution reaction (OER), producing a measurable photocurrent. Using TE-polarized light, we observe sharp peaks in the photocurrent and sharp dips in the photoreflectance at approximately 9° from normal incidence, which corresponds to the conditions under which there is good wavevector matching between the incident light and the lines in the grating. With light polarized parallel to the grating (i.e., TM), we excite the grating structure non-resonantly and there is no angular dependence in the photocurrent or photoreflectance. In order to quantify the lifetime of these hot carriers, we performed transient absorption spectroscopy of these plasmon resonant grating structures. Here, we observe one feature in the spectra corresponding to interband transitions and another feature associated with the plasmon resonant mode in the grating. Both features decay over a time scale of 1-2 ps. The spectral responses of grating structures fabricated with Ag, Al, and Cu are also presented.

We report on photoluminescence (PL) spectroscopy and transmission electron microscope imaging of suspended and substrate-supported flakes of the 2-D semiconductor MoS2 before and after exposure to 100-keV proton radiation with fluences of $6 \times 10^{13}$ , $6\times 10^{14}$ , and $6 \times 10^{15}$ p/cm2, respectively, and subsequent annealing. An indirect-to-direct bandgap transition is observed, which is preserved after annealing. This transition is accompanied by an unexpected increase in PL intensity after radiation exposure of multilayer samples, which is attributed to higher radiative efficiency of the direct-gap transition.

We explore the effect of charge density wave (CDW) on the in-plane thermoelectric transport properties of (PbSe)

We report cross-plane thermoelectric measurements of SnSe and SnSe2 films grown by the modulated element reactant (MER) approach. These materials exhibit ultralow cross-plane thermal conductivities, which are advantageous for thermoelectric energy conversion. The initially grown SnSe films have relatively low cross-plane Seebeck coefficients (−38.6 μV/K) due to significant unintentional doping originating from Se vacancies when annealed in nitrogen, as a result of the relatively high vapor pressure of Se. By performing postgrowth annealing at a fixed Se partial pressure (300 °C for 30 min using SnSe2 as the Se source in a sealed tube), a transition from SnSe to SnSe2 is induced, which is evidenced by clear changes in the X-ray diffraction patterns of the films. This results in a 16-fold increase in the cross-plane Seebeck coefficient (from −38.6 to −631 μV/K) after Se annealing due to both the SnSe-to-SnSe2 transition and the mitigation of unintentional doping by Se vacancies. We also observe a correspond...

In outer space down to the altitudes routinely flown by larger aircrafts, radiation can pose serious issues for microelectronics circuits. The 88-Inch Cyclotron at Lawrence Berkeley National Laboratory is a sector-focused cyclotron and home of the Berkeley Accelerator Space Effects Facility, where the effects of energetic particles on sensitive microelectronics are studied with the goal of designing electronic systems for the space community. This paper describes the flexibility of the facility and its capabilities for testing the bombardment of electronics by heavy ions, light ions, and neutrons. Experimental capabilities for the generation of neutron beams from deuteron breakups and radiation testing of carbon nanotube field effect transistor will be discussed.

We observe bright electroluminescence from suspended carbon nanotube (CNT) field effect transistors (FETs) under extremely low applied electrical powers (∼nW). Here, light emission occurs under positive applied gate voltages, with the FET in its “off” state. This enables us to apply high bias voltages (4 V) without heating the CNT. Under these conditions, we observe light emission at currents as small as 1 nA and corresponding electrical powers of 4nW, which is 3 orders of magnitude lower than previous studies. Thermal emission is ruled out by monitoring the G band Raman frequency, which shows no evidence of heating under these small electrical currents. The mechanism of light emission is understood on the basis of steep band bending that occurs in the conduction and valence band profiles at the contacts, which produces a peak electric field of 500 kV/cm, enabling the acceleration of carriers beyond the threshold of exciton emission. The exciton-generated electrons and holes are then accelerated in this f...

We report spectroscopic measurements of the local electric fields and local charge densities at electrode surfaces using graphene-enhanced Raman spectroscopy (GERS) based on the Stark-shifts of surface-bound molecules and the

It has been known for several decades that defects are largely responsible for the catalytically active sites on metal and semiconductor surfaces. However, it is difficult to directly probe these active sites because the defects associated with them are often relatively rare with respect to the stoichiometric crystalline surface. In the work presented here, we demonstrate a method to selectively probe defect-mediated photocatalysis through differential alternating current (ac) photocurrent (PC) measurements. In this approach, electrons are photoexcited from the valence band to a relatively narrow distribution of subband gap states in TiO

We report cross-plane thermoelectric measurements of SnSe and SnSe

The coupling interface between optical fiber and photonics chip waveguide is critical for optical coupling efficiency. Grating coupling offers better fiber mis-alignment tolerance, but it was limited by narrow coupling bandwidth which might not able to meet wavelength-division multiplexing requirement. Edge coupling offers broad bandwidth capability but suffers lower coupling efficiency (C.E.) for hifher fiber mis-alignment tolerance. In order to obtain high coupling efficiency and broad coupling bandwidth, we proposed multiple taper waveguide design for edge coupling PIC application. In this design, using two tapers with splitter waveguide, it enhances the overall coupling efficiency but not increase the complexity of the assembly process. According to simulation result, 1.5 dB higher coupling efficiency was obtained than traditional single taper design which is also able to accommodate higher fiber alignment tolerance. It is easily to be implemented in current commercial silicon photonics CMOS technology and can be worked with single mode fiber to provide better coupling efficiency and higher mis-alignment tolerance

We report a novel approach to probe the local ion concentration at graphene/water interfaces using in situ Raman spectroscopy. Here, the upshifts observed in the G band Raman mode under applied electrochemical potentials are used to determine the charge density in the graphene sheet. For voltages up to ±0.8 V vs. NHE, we observe substantial upshifts in the G band Raman mode by as much as 19 cm-1, which corresponds to electron and hole carrier densities of 1.4 × 1013 cm-2 and Fermi energy shifts of ±430 meV. The charge density in the graphene electrode is also measured independently using the capacitance-voltage characteristics (i.e., Q = CV), and is found to be consistent with those measured by Raman spectroscopy. From charge neutrality requirements, the ion concentration in solution per unit area must be equal and opposite to the charge density in the graphene electrode. Based on these charge densities, we estimate the local ion concentration as a function of electrochemical potential in both pure DI water and 1 M KCl solutions, which span a pH range from 3.8 to 10.4 for pure DI water and net ion concentrations of ±0.7 mol L-1 for KCl under these applied voltages.

Black phosphorus (BP) has been recently rediscovered as an elemental two-dimensional (2D) material that shows promising results for next generation electronics and optoelectronics because of its intrinsically superior carrier mobility and small direct band gap. In various 2D field-effect transistors (FETs), the choice of metal contacts is vital to the device performance, and it is a major challenge to reach ultralow contact resistances for highly scaled 2D FETs. Here, we experimentally show the effect of a work function tunable metal contact on the device performance of BP FETs. Using palladium (Pd) as the contact material, we employed the reaction between Pd and H2 to form a Pd–H alloy that effectively increased the work function of Pd and reduced the Schottky barrier height (ΦB) in a BP FET. When the Pd-contacted BP FET was exposed to 5% hydrogen concentrated Ar, the contact resistance (Rc) improved between the Pd electrodes and BP from ∼7.10 to ∼1.05 Ω·mm, surpassing all previously reported contact res...

Carbon nanotube (CNT) field effect transistors (FETs) are anticipated to provide a viable alternative to silicon as CMOS technology begins to reach ultimate scaling lengths. With high carrier mobilities, current density, and tunable bandgaps, CNTs can allow for the growth of the transistor industry beyond traditional materials. However, these nanotubes are sensitive to their surroundings and require a protective passivation layer to isolate them from contamination by their external environment. Thin parylene films have been shown to be a practical passivation layer for CNT devices as a flexible, chemically inert, high dielectric strength, pinhole-free material. In this paper, we perform electrical characterization of single suspended CNT FETs before and after passivation with parylene-C thin-film deposition. Analysis shows moderate changes in the threshold voltage, subthreshold slope, and ON/OFF ratio with parylene passivation, indicating minimal effect on the CNT FET overall performance.

We report plasmon resonant excitation of hot electrons in a metal based photocatalyst in the oxygen evolution half reaction in aqueous solution. Here, the photocatalyst consists of a 100-nm thick Au film deposited on a corrugated silicon substrate. In this configuration, hot electrons photoexcited in the metal are injected into the solution, ultimately reversing the water oxidation reaction (O2 + 4H+ + 4e− ⇋ 2H2O) and producing a photocurrent. In order to amplify this process, the gold electrode is patterned into a plasmon resonant grating structure with a pitch of 500 nm. The photocurrent (i.e., charge transfer rate) is measured as a function of incident angle using 633 nm wavelength light. We observe peaks in the photocurrent at incident angles of ±9° from normal when the light is polarized parallel to the incident plane (p-polarization) and perpendicular to the lines on the grating. Based on these peaks, we estimate an overall plasmonic gain (or amplification) factor of 2.1× in the charge transfer rate. At these same angles, we also observe sharp dips in the photoreflectance, corresponding to the condition when there is wavevector matching between the incident light and the plasmon mode in the grating. No angle dependence is observed in the photocurrent or photoreflectance when the incident light is polarized perpendicular to the incident plane (s-polarization) and parallel to the lines on the grating. Finite difference time domain simulations also predict sharp dips in the photoreflectance at ±9°, and the electric field intensity profiles show clear excitation of a plasmon-resonant mode when illuminated at those angles with p-polarized light.

We report in-plane thermoelectric measurements of Bi2Te3 nanoplates, a typical topological insulator with Dirac-like metallic surface states, grown by chemical vapor deposition. The as-grown flakes exposed to ambient conditions exhibit relatively small thermopowers around −34 μV/K due to unintentional surface doping (e.g., gas adsorption and surface oxidation). After removal of the unintentional surface doping and surface passivation by deposition of 30 nm of Al2O3 using atomic layer deposition (ALD), the Seebeck coefficient of these flakes increases by a factor of 5× to −169 μV/K. Here, we believe that the ALD-based surface passivation can prevent the degradation of the thermoelectric properties caused by gas adsorption and surface oxidation processes, thus, reducing the unintentional doping in the Bi2Te3 and increasing the Seebeck coefficient. The high surface-to-volume ratio of these thin (∼10 nm thick) nanoplates make them especially sensitive to surface doping, which is a common problem among nanomaterials in general. An increase in the sample resistance is also observed after the ALD process, which is consistent with the decrease in doping.

We report photoluminescence (PL) spectroscopy of air-suspended and substrate-supported molybdenum disulfide (MoS2) taken before and after exposure to proton radiation. For 2-, 3-, and 4-layer MoS2, the radiation causes a substantial (>10×) suppression of the indirect bandgap emission, likely due to a radiation-induced decoupling of the layers. For all samples measured (including the monolayer), we see the emergence of a defect-induced shoulder peak at around 1.7 eV, which is redshifted from the main direct bandgap emission at 1.85 eV. Here, defects induced by the radiation trap the excitons and cause them to be redshifted from the main direct band emission. After annealing, the defect-induced sideband disappears, but the indirect band emission remains suppressed, indicating a permanent transition into a direct bandgap material. While suspended 2-, 3-, and 4-layer MoS2 show no change in the intensity of the direct band emission after radiation exposure, substrate-supported MoS2 exhibits an approximately 2-...

Small diameter single wall carbon nanotubes are desirable for various physical and electrical properties of carbon nanotubes. Here, we report the sensitivities of parameters and the optimal conditions for small diameter carbon nanotube growth by chemical vapor deposition (CVD). These results were obtained using the Taguchi method, which is commonly used to find the optimal parameters of various processes. The possible parameter ranges given by the experimental equipment and laboratory conditions, we attempted several times to determine the proper ranges, using photoluminescence (PL) imaging to determine the exact positions of suspended carbon nanotubes on the quartz substrates after synthesis. The diameters of the carbon nanotubes were then determined from the radial breathing modes (RBM) using Raman spectroscopy with a 785nm wavelength laser. Among the 4 major parameters listed above, we concluded that the temperature was the most significant parameter in determining carbon nanotube diameter, hydrogen flow rate was the second most significant, the ethanol and argon gas flow rate was the third, and finally time was the least significant factor.

The dominant diffusing species in the formation of amorphous interlayer between Gd and Si thin films have been determined by a Mo cluster marker experiment. Multilayered metal thin films were deposited on (001) Si in an ultrahigh vacuum electron beam evaporator. The positions of the Mo cluster markers relative to the Si substrate, before and after heat treatment, were determined by transmission electron microscopy and energy dispersive x-ray as well as autocorrelation function analysis. The displacement of the Mo cluster markers in the amorphous interlayer during the Gd–Si interdiffusion indicates that Si atoms constitute the dominant diffusing species during the growth of the amorphous interlayer.

The effect of Cu doped into Pt spacer layers on magneto-optical properties for sputtered Pt/Co multilayers has been investigated. The polar Kerr rotation θk oscillates with the content of Cu in Pt spacer layers, and an obvious enhancement of θk at the first and second peaks on its oscillation curve can be seen. The numerical calculation indicates that the components of conductivity tensor in PtCu layers make a simultaneous oscillation with the polar Kerr rotation. These behaviors may result from the spin polarization in PtCu layers.

In Appl. Phys. Lett. 67, 1990 (1995), we proposed a particularly simple four‐domain (4‐D) twisted nematic(TN) liquid crystal display(LCD) device, which is composed of two left‐handed and two right‐handed TN subpixels. The two members of each pair of same handedness subpixels are rotated 180° with respect to each other, resulting in four domains that spatially average one another optically to provide a wide angle of viewing with no gray scale inversion. The optical performance of the 4‐D TN LCD was confirmed by studies of a test cell fabricated by a two‐step SiOx oblique evaporation technique. In this article, we report the realization of our four‐domain TN display by a reverse rubbing technique that should be suitable for mass production in the display industry. The optical simulation of our 4‐D TN cell was performed and the effect of disclinations at subpixel boundaries on display contrast investigated. A simple model was developed to evaluate the stability of our 4‐D structure.

Layered transition metal dichalcogenides, such as MoS2, have recently emerged as a promising material system for electronic and optoelectronic applications. The two-dimensional nature of these materials enables facile integration for vertical device design with novel properties. Here, we report highly efficient photocurrent generation from vertical MoS2 devices fabricated using asymmetric metal contacts, exhibiting an external quantum efficiency of up to 7%. Compared to in-plane MoS2 devices, the vertical design of these devices has a much larger junction area, which is essential for achieving highly efficient photovoltaic devices. Photocurrent and photovoltage spectra are measured over the photon energy range from 1.25 to 2.5 eV, covering both the 1.8 eV direct K-point optical transition and the 1.3 eV Σ-point indirect transition in MoS2. Photocurrent peaks corresponding to both direct and indirect transitions are observed in the photocurrent spectra and exhibit different photovoltage–current characteristics. Compared to previous in-plane devices, a substantially shorter photoresponse time of 7.3 μs is achieved due to fast carrier sweeping in the vertical devices, which exhibit a −3 dB cutoff frequency of 48 kHz.

We report photoluminescence (PL) imaging and spectroscopy of air-suspended carbon nanotubes (CNTs) before and after exposure to a brief (20 s) UV/ozone treatment. These spectra show enhanced PL intensities in 10 out of 11 nanotubes that were measured, by as much as 5-fold. This enhancement in the luminescence efficiency is caused by oxygen defects which trap excitons. We also observe an average 3-fold increase in the D-band Raman intensity further indicating the creation of defects. Previous demonstrations of oxygen doping have been carried out on surfactant-coated carbon nanotubes dissolved in solution, thus requiring substantial longer ozone/UV exposure times (∼15 h). Here, the ozone treatment is more efficient because of the surface exposure of the air-suspended CNTs. In addition to enhanced PL intensities, we observe narrowing of the emission linewidth by 3–10 nm. This ability to control and engineer defects in CNTs is important for realizing several optoelectronic applications such as light-emitting ...

Radially and azimuthally oriented liquid crystal (LC) alignment patterns, which are hard to achieve by conventional LC alignment techniques, have been realized by exposure of polyimide surfaces to linearly polarized ultraviolet radiation. These surfaces can be used to realize some special LC configurations which are useful for LC basic research and applications.

A structure for an electro‐optically controlled liquid crystal diffraction grating is proposed, which can dramatically simplify the fabrication process of liquid crystal optical gratings. The structure consists of two alternating stripes. Each stripe is a hybrid liquid crystal cell with adjacent stripes oriented perpendicularly. This kind of electro‐optically controlled diffraction grating in principle gives 100% diffraction efficiency and no polarization direction dependence. The detailed fabrication process is presented.

A particularly simple four‐domain (4‐D) twisted nematic (TN) liquid crystal display (LCD) device is proposed, which is composed of two left‐handed TN and two right‐handed TN subpixels. One of each pair of same handedness subpixels is rotated 180° with respect to the other, resulting in four domains that spatially average one another optically to provide a wide angle of viewing with no gray scale inversion. The detailed fabrication process is presented for a two step SiOx oblique evaporation technique used to realize this 4‐D TN LCD. A reverse rubbed polyamide fabrication process has also been successfully used and will be presented in the full length article. Here we present the complete viewing angle and contrast ratio data for a simple and successful 4‐D TN LCD cell.