micro-Raman Spectroscopy Measurements Research Articles

Fabrication of quantum-dot devices in graphene

We describe our recent experimental results on the fabrication of quantum-dot devices in a graphene-based two-dimensional system. Graphene samples were prepared by micromechanical cleavage of graphite crystals on a SiO2/Si substrate. We performed micro-Raman spectroscopy measurements to determine the number of layers of graphene flakes during the device fabrication process. By applying a nanofabrication process to the identified graphene flakes, we prepared a double-quantum-dot device structure comprising two lateral quantum dots coupled in series. Measurements of low-temperature electrical transport show the device to be a series-coupled double-dot system with varied interdot tunnel coupling, the strength of which changes continuously and non-monotonically as a function of gate voltage.

Effects of mechanical stress on thermal microactuator performance

Mechanical stresses on microsystems die induced by packaging processes and varying environmental conditions can affect the performance and reliability of microsystems devices. Thermal microactuators and stress gauges were fabricated using the Sandia five-layer SUMMiT surface micromachining process and diced to fit in a four-point bending stage. The sample dies were tested under tension and compression at stresses varying from −250 MPa, compressive, to 200 MPa, tensile. Stress values were validated by both on-die stress gauges and micro-Raman spectroscopy measurements. Thermal microactuator displacement is measured for applied currents up to 35 mA as the mechanical stress is systematically varied. Increasing tensile stress decreases the initial actuator displacement. In most cases, the incremental thermal microactuator displacement from the zero current value for a given applied current decreases when the die is stressed. Numerical model predictions of thermal microactuator displacement versus current agree with the experimental results. Quantitative information on the reduction in thermal microactuator displacement as a function of stress provides validation data for MEMS models and can guide future designs to be more robust to mechanical stresses.

Synthesis of the Higher-Order Diamondoid Hexamantane Using Low-Temperature Plasmas Generated in Supercritical Xenon

Diamondoid molecules were synthesized from adamantane (C10H16) using low-temperature plasmas generated in supercritical xenon. The carbon content of the synthesized materials was verified by energy dispersive X-ray spectroscopy, while micro-Raman spectroscopy measurements confirmed that the synthesized materials contained sp3 bonds, the features in the Raman spectra being similar to those found in the Raman spectra of higher order diamondoids. Mass peaks at m/z = 396 were most abundant and might be attributed to C30H36 isomers of hexamantane. The synthesis of this particular type of diamondoid is explained by the fewer necessary cleavages of C–C bonds or C–H occurring to form the diamondoid.

Spectroscopic analyses of 0.5Li[Ni 0.8Co 0.15Zr 0.05]O 2–0.5Li[Li 1/3Mn 2/3]O 2 composite cathodes for lithium rechargeable batteries

In this work we have investigated the lithium ion intercalation behavior in wet chemical synthesized 0.5Li[Ni 0.8Co 0.15Zr 0.05]O 2–0.5Li[Li 1/3Mn 2/3]O 2 composite cathodes. Impedance spectroscopy in conjunction with micro-Raman spectroscopy and X-ray photoelectron spectroscopy measurements are performed to understand the lithium ion intercalation behavior in these composite cathodes. The Rietveld refinement of the X-ray diffractograms of the cathode powder indicates a hexagonal layered structure and minimal transitional metal cation disorder between lithium and (transition) metal layers. Probing the localized region by micro-Raman spectroscopy, it is found that in virgin cathode, monoclinic Li[Li 1/3Mn 2/3]O 2 forms composite with hexagonal layered Li[Ni 0.8Co 0.15Zr 0.05]O 2 component. Interestingly, these cathodes transform into a hexagonal layered structure upon charging ∼ 4.8 V. The galvanostatic charge–discharge characteristics of these composite cathode is reported in the cut-off voltage limits of 4.8–1.45 V. Detailed X-ray photoelectron spectroscopy analyses of the valence states of the constituent elements are performed in virgin, fully charged and partial discharged states of the cathodes. Through the XPS analyses, the charge and discharge capacities of these cathodes are found to be due to Ni 3+/Ni 4+, Co 3+/Co 4+ and Ni 2+/Ni 3+ redox couples. The capacity of these composite cathodes is deteriorated when cycled repeatedly between the voltage limits of 4.8–1.45 V. The probable reasons for such findings are discussed.

Structure, optical properties and thermal stability of pulsed sputter deposited high temperature HfO x/Mo/HfO 2 solar selective absorbers

Solar selective coatings of HfO x /Mo/HfO 2 were deposited on copper (Cu) and stainless steel (SS) substrates using a magnetron sputtering system. The HfO x and HfO 2 layers were deposited from the sputtering of Hf target in Ar+O 2 plasma using an asymmetric bipolar-pulsed direct current generator, whereas the Mo layer was deposited from the sputtering of Mo target in the Ar plasma. The optimized HfO x /Mo/HfO 2 multilayer absorber on Cu substrate exhibited high solar absorptance ( α=0.905–0.923) and low thermal emittance ( ε 82 °C =0.07–0.09). Similarly, on SS substrates the optimized coatings exhibited α and ε 82 °C in the ranges of 0.902–0.917 and 0.15–0.17, respectively. The X-ray diffraction data showed that the HfO x /Mo/HfO 2 coating consists of tetragonal and monoclinic phases of HfO 2, which was confirmed by micro-Raman spectroscopy data. The bonding structure of the HfO x and the HfO 2 layers were confirmed using X-ray photoelectron spectroscopy data. The optical constants ( n and k), measured using spectroscopic ellipsometry, showed that the top HfO 2 layer acts as an antireflection coating and the bottom two layers (HfO x and Mo) are the main absorber layers. The analysis of the spectroscopic ellipsometric data indicated that the band gap of HfO x and HfO 2 layers were 3.90 and 5.82 eV, respectively, indicating non-stoichiometric nature of HfO x . In order to study the thermal stability of the HfO x /Mo/HfO 2 coatings, they were subjected to heat treatment in air and vacuum at different temperatures ( T A). The HfO x /Mo/HfO 2 coatings deposited on Cu substrates were thermally stable up to 400 °C for 2 h in air. Addition of a thin Mo interlayer (40 nm) in the HfO x /Mo/HfO 2 coating (i.e., Mo/HfO x /Mo/HfO 2) deposited on Cu substrates exhibited high solar selectivity ( α/ ε) of 0.872/0.09 even after heat-treatment in air up to 500 °C for 2 h. At T A=525 °C, the solar selectivity decreased drastically ( α/ ε=0.761/0.35) due to the formation of MoO 2, MoO 3 and HfMo 2O 8 phases. The Mo/HfO x /Mo/HfO 2 coatings deposited on SS substrates showed no significant changes in α and ε values after annealing at 500 °C in air and at 800 °C in vacuum. These results were confirmed by micro-Raman spectroscopy measurements, which showed the compositional stability of these coatings up to 500 °C in air and 800 °C in vacuum.

Direct-bandgap luminescence at room-temperature from highly-strained Germanium nanocrystals

Efficient room-temperature luminescence at optical telecommunication wavelengths and originating from direct band-to-band recombination has been observed in tensile-strained germanium nanocrystals synthesized by mechanical grinding techniques. Selected area electron diffraction, micro-Raman and optical-absorption spectroscopy measurements indicate high tensile-strains while combined photoluminescence spectroscopy, excitation-power evolution and time-resolved measurements suggest direct band-to-band recombination. Such band-engineered germanium nanocrystals offer great possibilities for silicon-photonics integration due to their superb light-emission properties, facile fabrication and compatibility with standard microelectronic processes.

Unique Synthesis of Few-Layer Graphene Films on Carbon-Doped Pt83Rh17 Surfaces

We report a unique synthesis of single- and few-layer graphene films on carbon-doped Pt(83)Rh(17) surfaces by surface segregation and precipitation. The ultrathin graphene films were characterized by atomic force microscopy, Auger electron spectroscopy, and micro-Raman spectroscopy measurements, providing evidence of graphene film thickness and structural quality. The G and 2D band intensity images from micro-Raman spectroscopy measurements confirm that the graphene films with different coverage have very limited defects. Additionally, the 2D band peak can be well-fitted by a single Lozentian peak, indicating that graphene films are characteristic of single layer graphene. Graphene film thickness can be determined by analysis of Auger spectra, indicating that graphene films after 850 degrees C annealing mainly consist of monolayer graphene. By precise adjustment of annealing temperature, graphene film thickness and area size can be controlled and uniform large-area single-layer and double-layer graphene can be achieved.

Tuning surface properties in photosensitive polyimide. Material design for high performance organic thin-film transistors

A photosensitive polyimide (PSPI) is synthesized to serve as the gate dielectric in organic thin-film transistors (OTFTs). A method to control the charge density within the channel of the OTFT using the PSPI on a silicon dioxide gate dielectric is proposed. The surface energy, surface carriers, and capacitance of gate dielectrics can be tuned by varying irradiation doses of UV light on the PSPI surface to create ultrahigh performance pentacene-based OTFTs with an average field-effect mobility (μ) of above 6.0 cm2/Vs. The correlation between the surface energy of the PSPI films and the μ value of OTFT devices is investigated. Moreover, the impact of gate-dielectric properties on the structural properties of pentacene films is closely related to the electrical characteristics of OTFTs. The results of contact angle, X-ray diffraction, electrostatic force microscopy, and micro-Raman spectroscopy measurements indicate that the surface energy and doping carriers coming from the PSPI film play important roles in the carrier transport of pentacene film for OTFT applications.

Exciton-phonon coupling in individual GaAs nanowires studied using resonant Raman spectroscopy

The Fr\"ohlich coupling strength of individual GaAs nanowires is investigated by resonant micro-Raman spectroscopy measurements near the direct bandgap ${E}_{g}$. Large 2LO/1LO intensities up to 5.7 are observed in an individual GaAs nanowire. A 2LO resonance profile of the GaAs nanowire agrees well with a two-phonon-scattering model, suggesting excitonic scattering. These results advance the understanding of electron-phonon coupling and exciton scattering in quasi-one-dimensional systems and in GaAs at ${E}_{g}$, allowing for the development and optimization of nanowire optoelectronic devices.

Characteristics of SiOx-cored composite nanowires with Pd shell layers

This paper describes the fabrication of Pd-coated SiOx nanowires via a simple approach and reports the effects of subsequent thermal annealing. While the as-synthesized Pd-coated SiOx nanowires exhibited smooth one-dimensional (1D) nanostructures, the thermal annealing induced a rough surface. X-ray diffraction, selected area electron diffraction, and lattice-resolved transmission electron microscopy coincidentally revealed that the assynthesized Pd-coated SiOx nanowires were comprised of a cubic Pd phase, whereas annealing generated a tetragonal PdO phase. Micro-Raman spectroscopy measurements showed the presence of 630 cm−1-peak, which can be assigned to the B1g mode of single crystal PdO, whether samples had been annealed or not. Although both Pd and PdO phases did not exhibit PL emission, the PL intensity of core SiOx nanowires has been significantly reduced by the Pd-coating, presumably due to the covering effect.

Micro-Raman spectroscopy studies of bulk and thin films of CuInTe2

Micro-Raman spectroscopy measurements were made on polycrystalline and amorphous thin films of CuInTe2 as well as bulk polycrystalline CuInTe2. Various vibrational modes exhibited by the bulk and polycrystalline thin films were attributed to those expected for single crystal CuInTe2. Raman spectra of amorphous films presented a broad spectrum, decomposition of which revealed the presence of elemental tellurium on the film surface. Laser-induced changes on CuInTe2 thin films were studied by acquiring spectra with higher laser beam power. Modes due to tellurium appeared when the spectra were acquired during laser–sample interaction, indicating tellurium segregation. The Raman spectra measured from polycrystalline films during high laser power irradiation did not show decrease in the intensity of the A1 mode of CuInTe2 in spite of loss of tellurium from the lattice. This has been interpreted as related to an increased contribution from the undistorted subsurface CuInTe2 region at higher excitation power.

Understanding the role of Zr 4+ cation in improving the cycleability of LiNi 0.8Co 0.15Zr 0.05O 2 cathodes for Li ion rechargeable batteries

Lithium nickel cobalt oxide (LNCO) has been considered as one of the most attractive cathode materials for lithium rechargeable batteries. The reported discharge capacities of LNCO are scattered and these cathodes are prone to capacity fading upon repeated charge–discharge cycling. Although, inert cation modified LNCO imparts better structural stability and partially circumvent the fading problem; the role played by such dopant in improving the electrochemical characteristics remains unclear. In the present work, we have demonstrated Zr 4+ modified LNCO yields decent discharge capacity and excellent cycleability. X-ray Rietveld analyses in conjunction with micro-Raman and X-ray photoelectron spectroscopy measurements were performed to understand the role of Zr 4+ cation to improve the cycleability of LNCO cathode. Based on the analyses of XRD refinement and spectroscopy data we have argued that the positive role of Zr 4+ dopant are realized through its preferential occupancy in lithium site (of hexagonal layered LNCO) to impart structural stability during charge–discharge cycling. Additionally, with repeated charge–discharge cycles probably part of Zr migrates to the electrode surface to form a passive layer to inhibit Ni 3+ → Ni 2+ reduction to improve the cycleability.

Optical properties and thermal stability of pulsed-sputter-deposited Al xO y/Al/Al xO y multilayer absorber coatings

Spectrally selective Al x O y /Al/Al x O y multilayer absorber coatings were deposited on copper (Cu) and molybdenum (Mo) substrates using a pulsed sputtering system. The Al targets were sputtered using asymmetric bipolar-pulsed DC generators in Ar+O 2 and Ar plasmas to deposit an Al x O y /Al/Al x O y coating. The compositions and thicknesses of the individual component layers were optimized to achieve high solar absorptance ( α=0.950–0.970) and low thermal emittance ( ε=0.05–0.08). The X-ray diffraction data in thin film mode showed an amorphous structure of the Al x O y /Al/Al x O y coating. The X-ray photoelectron spectroscopy data of the Al x O y /Al/Al x O y multilayer absorber indicated that the Al x O y layers present in the coating were non-stoichiometric. The optical constants ( n and k) of the multilayer absorber were determined from the spectroscopic ellipsometric data. Drude's free-electron model was used for generating the theoretical dispersion of optical constants for Al films, while the Tauc–Lorentz model was used for modeling optical properties of the dielectric Al x O y layers. In order to study the thermal stability of the Al x O y /Al/Al x O y coatings, they were subjected to heat treatment (in air and vacuum) at different temperatures and durations. The multilayer absorber deposited on Cu substrates exhibited high solar selectivity ( α/ ε) of 0.901/0.06 even after heat-treatment in air up to 400 °C for 2 h. At 450 °C, the solar selectivity decreased significantly on Cu substrates (e.g., α/ ε=0.790/0.07). The coatings deposited on Mo substrates were thermally stable up to 800 °C in vacuum with a solar selectivity of 0.934/0.05. The structural stability of the absorber coatings heat treated in air (up to 400 °C) and vacuum (up to 800 °C) was confirmed by micro-Raman spectroscopy measurements. Studies on the accelerated aging tests suggested that the absorber coatings on Cu were stable in air up to 75 h at 300 °C and the service lifetime of the multilayer absorber was predicted to be more than 25 years. Further, the activation energy for the degradation of the multilayer absorber heat treated for longer durations in air is of the order of 64 kJ/mol.

Annealing effect on the chemical structure of diamondlike carbon

The effect of annealing in an ultrahigh vacuum on the chemical structure of diamondlike carbon (DLC) was investigated using photoelectron spectroscopy, thermal desorption spectroscopy, electrical resistivity, and micro-Raman spectroscopy measurements. The line shapes of the C 1s photoelectron spectra depended on annealing temperature. The relative intensities of four chemical components in the spectra were quantitatively evaluated: sp3 carbon with carbon-carbon bonds (C–C sp3 carbon), sp2 carbon with carbon-carbon bonds (C–C sp2 carbon), sp2 carbon with hydrogen-carbon bonds (H–C sp2 carbon), and sp3 carbon with hydrogen-carbon bonds (H–C sp3 carbon). The variation of the ratio of the components demonstrated that hydrogen in DLC is emitted to the outside in between 450 and 600 °C, and the remaining DLC is graphized above 600 °C. The increase in the asymmetry of the C 1s spectra and the decrease in the electrical resistivity of the DLC film with annealing temperature agree with the picture that the H–C bonds in DLC produces large free spaces in the structure, which inhibit conductive routes and lead to high electrical resistivity.

Bandgap renormalization in titania modified nanostructured tungsten oxide thin films prepared by pulsed laser deposition technique for solar cell applications

Pure and titania modified WO3 films are prepared using pulsed laser ablation technique in an oxygen ambient of 0.12mbar at a substrate temperature of 873K. Titania incorporation effects on the microstructure, optical, and electrical properties on the tungsten oxide films are systematically investigated using techniques such as x-ray diffraction, atomic force microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, micro-Raman spectroscopy, and UV-visible absorption spectroscopy measurements. The resistivity measurements of the pure and titania modified WO3 films are done at room temperature. The variation of resistivity with temperature for the range of 170–450K is also investigated. The microstructural analysis indicates that titania addition strongly perturbs the tungsten oxide lattice and suppresses the grain growth. Optical measurements revealed a bandgap renormalization in tungsten oxide films for higher titania concentrations. Bandgap values of the films decrease from 3.17eV for pure WO3 to 2.7eV for 10wt% TiO2 modified WO3 films. The shifting of photoresponse of WO3 films to visible range by titania incorporation throw light on the feasibility of their application in visible-light-driven photocatalysis and in solar cells. All the films show semiconducting behavior in their temperature dependence of resistivity.

Highly crystallized silicon films grown on glass without amorphous incubation layers by inductively coupled plasma chemical vapor deposition

Highly crystallized silicon films were deposited on glass by inductively coupled plasma chemical vapor deposition using a SiH 4/H 2 mixture as the source gas at a substrate temperature of 350 °C. The micro-Raman and X-ray diffraction spectroscopy (XRD) measurements demonstrate that at the constant SiH 4 dilution ratio of 10%, the crystallinity of the resultant films increases in concomitance with the change of the preferred orientation from (1 1 1) to (2 2 0) as the total working pressure increases from 20 to 40 Pa. With further increase in the total working pressure to 60 Pa, the crystallinity decreases significantly and the preferred orientation remains at (2 2 0). More studies on film microstructures were performed using spectroscopic ellipsometry and a scanning electron microscope. It was found that film deposition could be achieved without the amorphous incubation layer when the total working pressure was 40 Pa.

Submicrometre periodic surface structures in InP induced by nanosecond UV laser pulses

We report fabrication of submicrometre size laser-induced periodic surface structures (ripples) on single crystalline InP by nanosecond (ns) pulsed Nd : YAG laser beam irradiation of fourth harmonic wavelength (266 nm) in HF electrolyte. The ripples are orientated parallel to the laser polarization direction and power spectral density analysis reveals reduction in the spatial period of the ripples with increasing number of laser shots. The formation of periodic structures in the presence of electrolyte is empirically explained on the basis of photoelectrochemical etching and variation of periodicity with refractive index change on laser energy and number of laser pulses. From the analysis of energy dispersive x-ray, photoluminescence (PL) and micro-Raman spectroscopy measurements on the rippled surface we conclude that the ripple structures are capped with a thin layer of In2O3. Further, a blue shift of 0.328 eV compared with the band-edge luminescence of InP is estimated from the PL spectrum of the structure fabricated with 200 laser shots. The blue shift of the PL peak is attributed to the quantum confinement effect in the nanometre size structures in the rippled surface. Micro-Raman spectra show good crystalline quality of the surface at lower number of laser shots and its degradation caused by oxidation at the higher number of shots.

Synthesis by Wet Chemical Method and Characterization of Nanocrystalline ZnO Doped with Fe2O3

Nanocrystalline samples of ZnO doped with Fe2O3 were synthetized by wet chemical method. The series of ZnO nanosized samples in the wide range of Fe2O3 concentration (from 5 wt.% to 95 wt.%) was prepared by precipitation from nitrate solutions using ammonia. The phase composition of the samples was determined using X-ray diffraction measurements. The phases of hexagonal ZnO, and/or rhombohedric Fe2O3, and/or ZnFe2O4 were identified. The mean crystalline size of nanocrystals, determined with the use of Scherrer’s formula, varied from 8 to 52 nm. The preliminary micro-Raman spectroscopy measurements were performed. The observed features are typical of Fe doped ZnO nanoparticles. The magnetic measurements revealed the presence of different types of magnetic behavior. For samples with high Fe2O3 contents (above 70 wt.%) the ferromagnetic ordering was observed at room temperature. For samples with lower Fe2O3 contents we observed the phenomenon of superparamagnetism above the blocking temperature.

Structure and optical properties of pulsed sputter deposited CrxOy∕Cr∕Cr2O3 solar selective coatings

Spectrally selective CrxOy∕Cr∕Cr2O3 multilayer absorber coatings were deposited on copper (Cu) substrates using a pulsed sputtering system. The Cr targets were sputtered using asymmetric bipolar-pulsed dc generators in Ar+O2 and Ar plasmas to deposit a CrxOy (bottomlayer)∕Cr∕Cr2O3 (top layer) coating. The compositions and thicknesses of the individual component layers have been optimized to achieve high absorptance (0.899–0.912) and low emittance (0.05–0.06). The x-ray diffraction data in thin film mode showed that the CrxOy∕Cr∕Cr2O3 coating consists of an amorphous phase; the Raman data of the coating, however, showed the presence of A1g and Eg modes, characteristic of Cr2O3. The x-ray photoelectron spectroscopy (XPS) data from near-surface region of the absorber suggested that the chemical state of Cr was in the form of Cr3+ and no phases of CrO2 and CrO3 were present. The experimental spectroscopic ellipsometric data have been fitted with theoretical models to derive the dispersion of the optical constants (n and k). The optical constants of the three layers indicate that the bottom two layers are the main absorber layers and the top Cr2O3 layer, which has higher oxygen content, acts as an antireflection coating. In order to study the thermal stability of the CrxOy∕Cr∕Cr2O3 coatings, they were subjected to heat treatment (in air and vacuum) at different temperatures and durations. The coating deposited on Cu substrates exhibited high solar selectivity (α∕ε) of 0.895∕0.06 even after heat treatment in air up to 300°C for 2h. At higher temperatures, the solar selectivity decreased significantly (e.g., α∕ε=0.855∕0.24 at 350°C in air), which is attributed to oxidation of Cr crystallites, increased surface roughness, and formation of CuO. The formation of CuO and the increase in Cr3+ vacancies due to the outward diffusion of Cr at higher annealing temperatures were confirmed by XPS. In the case of vacuum annealing, for temperatures greater than 500°C the outward diffusion of Cu was the dominating degradation mechanism. The microstructural stability of the absorber coatings heat treated in air (up to 325°C) and vacuum (up to 600°C) was confirmed by micro-Raman spectroscopy measurements. Studies on the accelerated aging tests indicated that the absorber coatings on Cu were stable in air up to 250h at 250°C with a solar selectivity of 0.898∕0.11.

Fabrication of ordered films of polystyrene–gold composites by a new flow-controlled method

A new method to form millimeter-sized colloidal crystals of polystyrene microsphere-gold composites has been developed. The experimental setup––a surface-treated microslide placed onto an inclined short burette that contains the mixture of microspheres and gold colloids––allows a controlled array growth at a steady, low rate. The velocity of the receding suspension can be controlled by both the angle of inclination of the burette and the degree of the opening of the stopcock. The dependence of the structure and the optical properties of the composite on the array growth rate, the volume fraction of microspheres, and the presence of a surfactant have been studied. Atomic force microscopy (AFM) measurements have shown that well-ordered hexagonal arrays are formed at low angles when suspensions with high volume fractions and gold nanoparticles are deposited at a very low rate. Micro-Raman spectroscopy measurements showed an enhancement effect brought about by the gold aggregates present in the sample. The high quality of the composite film obtained by this simple and yet efficient method makes it interesting for photonic applications.