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Recently, it has been shown how shift and ballistic currents in piezoelectric sillenite crystals Bi$_{12}$GeO$_{20}$ and Bi$_{12}$SiO$_{20}$ can be separated experimentally under the assumption that the shift component of the circular current is small. However, it has been claimed that the shift and ballistic currents cannot be quantified by this method, due to the magneto-photovoltaic effect caused either by the change of the crystal's spatial symmetry in the magnetic field or by the breaking of time-reversal symmetry. Presently, we report observations of photovoltaic currents in Bi$_{12}$SiO$_{20}$, excited by linearly- and circularly-polarized light under weak external magnetic field, as well as measurements of the corresponding photo-Hall signals. We demonstrate that the magneto-photovoltaic current constitutes a significant fraction of the measured current in the Hall direction for Bi$_{12}$SiO$_{20}$ under specific experimental conditions., Comment: 9 pages, 3 figures

In this study, we report a novel single-wall carbon nanotube (SWCNT)-TiO2 composite photoanode with enhanced thermal conductivity for photovoltaic applications. Thermal chemical vapor deposition technique was used for the growth of high quality SWCNTs. Structural and morphological properties of as synthesized SWCNT and SWCNT-TiO2 composites were inspected using ultra-high resolution transmission electron microscope (UHRTEM) and Raman spectrosctopy. Thermal conductivity of pure titania (TiO2) and SWCNT-TiO2 composites were estimated by an optothermal method. Photovoltaic performance of Dye-sensitized solar cell (DSSC) containing composite photoanodes was studied at various temperatures (ranging from room temperature-323K). We observed a notable decline in the overall photovoltaic performance of the DSSC containing pure TiO2 photoanode at elevated temperatures. Whereas, DSSC with composite photoanodes showed a significant improvement in overall photovoltaic performance consistently at room temperature as well as at elevated temperatures that is believed to be due to the existence of SWCNT and provides a conducting pathway to electrons in trap states and which are released at higher temperature and thus, quench recombination more effectively.

In recent years, BiFeO3 (BFO) thin films showed promising potential towards photovoltaic applications due to its superior ferroelectric properties and optical bandgap lies in the visible region. Among other wide bandgap ferroelectric photovoltaic (Fe-PV) materials (i.e. BaTiO3, LiNbO3 and Pb(Zr,Ti)O3), BFO exhibits large open circuit voltage (VOC), tunable output, and switchable photovoltaic effect. It has been reported that insertion of semiconductor layer to a conventional metal/ferroelectric/metal structure can result into novel heterostructures based photovoltaic cells. However, integration of novel carbon materials in ferroelectric photovoltaic devices has not been very popular. We have studied photovoltaic effect in In2O3-Sn2O3 (ITO)/reduced graphene oxide (rGO)/BiFeO3/Au (metal/semiconductor/ferroelectric/metal heterostructure). We have synthesized graphene oxide using modified hummers method. We spin coated GO solution on ITO substrate and dried overnight at 50oC to achieve thin films. Resultant GO thin films were annealed in NH3:H2 (3:1) atmosphere at 600oC. Electrical measurements confirmed n-type doping in thermally reduced GO (rGO) films. Whereas, BFO act as p-type semiconductor due to charge defects mainly, Oxygen vacancies. BFO films were grown using pulsed laser deposition technique. ITO/rGO/BFO/Au heterostructures shows a reasonable photovoltaic response. Such heterostructure shows short-cicuit current density of 86 µA/cm2. We believe that two built-in fields generated at Au/BFO and BFO/rGO interface are responsible for separation and transport of photo-excited electron-hole pairs. Our work may provide a new way to integrate carbon materials to ferroelectrics to produce heterojunction based solar cells.

Shift and ballistic photovoltaic currents, previously indistinguishable experimentally, can now be separated., The quantum phenomenon of shift photovoltaic current was predicted decades ago, but this effect was never observed directly because shift and ballistic currents coexist. The atomic-scale relaxation time of shift, along with the absence of a photo-Hall behavior, has made decisive measurement of shift elusive. Here, we report a facile, direct-current, steady-state method for unambiguous determination of shift by means of the simultaneous measurements of linear and circular bulk photovoltaic currents under magnetic field, in a sillenite piezoelectric crystal. Comparison with theoretical predictions permits estimation of the signature length scale for shift. Remarkably, shift and ballistic photovoltaic currents under monochromatic illumination simultaneously flow in opposite directions. Disentangling the shift and ballistic contributions opens the way for quantitative, fundamental insight into and practical understanding of these radically different photovoltaic current mechanisms and their relationship.

Tin titanate ($\mathrm{SnTi}{\mathrm{O}}_{3}$) has been notoriously impossible to prepare as a thin-film ferroelectric, probably because high-temperature annealing converts much of the $\mathrm{S}{\mathrm{n}}^{2+}$ to $\mathrm{S}{\mathrm{n}}^{4+}$. In the present paper, we show two things: first, perovskite phase $\mathrm{SnTi}{\mathrm{O}}_{3}$ can be prepared by atomic-layer deposition directly onto $p$-type Si substrates; and second, these films exhibit ferroelectric switching at room temperature, with $p$-type Si acting as electrodes. X-ray diffraction measurements reveal that the film is single-phase, preferred-orientation ferroelectric perovskite $\mathrm{SnTi}{\mathrm{O}}_{3}$. Our films showed well-saturated, square, and repeatable hysteresis loops of around $3\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/\mathrm{c}{\mathrm{m}}^{2}$ remnant polarization at room temperature, as detected by out-of-plane polarization versus electric field and field cycling measurements. Furthermore, photovoltaic and photoferroelectricity were found in $\mathrm{Pt}/\mathrm{SnTi}{\mathrm{O}}_{3}/\mathrm{Si}/\mathrm{SnTi}{\mathrm{O}}_{3}/\mathrm{Pt}$ heterostructures, the properties of which can be tuned through band-gap engineering by strain according to first-principles calculations. This is a lead-free room-temperature ferroelectric oxide of potential device application.

Room temperature ferroelectricity in pulsed laser deposited rare-earth doped hafnium oxide (HfO2) thin films is discussed. Maximum values of remnant polarizations (Pr) ~13.5 and 12 μC/cm2 along with coercive fields (EC) ~334 and 384 kV/cm are observed in 6 mol. % of rare-earth (Sm or Gd) doped-HfO2 thin films (Sm:HfO2 and Gd:HfO2), respectively. Piezoresponse force microscopy measurements confirmed ferroelectric nature of films by showing phase hysteresis and butterfly amplitude loops. It is noticed that wake-up cycles improved the remnant polarization and found to be necessary for the forming of well saturated hysteresis loops. Our results showed potential toward realization of future highly scaled non-volatile ferroelectric memories.

Thermal properties of single wall carbon nanotube sheets (SWCNT-sheets) are of significant importance in the area of thermal management, as an isolated SWCNT possesses high thermal conductivity of the value about 3000 W m(-1) K(-1). Here we report an indirect method of estimating the thermal conductivity of a nanometer thick suspended SWCNT-sheet by employing the Raman scattering technique. Tube diameter size is examined by the transmissions electron microscopy study. The Raman analysis of the radial breathing modes predicts narrow diameter size distribution with achiral (armchair) symmetry of the constituent SWCNTs. From the first order temperature coefficient of the A1g mode of the G band along with the laser power dependent frequency shifting of this mode, the thermal conductivity of the suspended SWCNT-sheet is estimated to be about ∼18.3 W m(-1) K(-1). Our theoretical study shows that the thermal conductivity of the SWCNT-sheet has contributions simultaneously from the intratube and intertube thermal transport. The intertube thermal conductivity (with contributions from the van der Waals interaction) is merely around 0.7 W m(-1) K(-1), which is three orders smaller than the intratube thermal conductivity, leading to an abrupt decrease in the thermal conductivity of the SWCNT-sheet as compared to the reported value for isolated SWCNT.

Here, we report the observation of ferroelectric and ferroelastic nanodomains in (110)-oriented BiFeO3 (BFO) thin films epitaxially grown on low symmetric (100) NdGaO3 (NGO) substrate. We observed long range ordering of ferroelectric 109° stripe nanodomains separated by periodic vertical domain walls in as-grown 130 nm thick BFO films. The effect of La0.67Sr0.33CoO3 (LSCO) conducting interlayer on domain configurations in BFO/NGO film was also observed with relatively short range-ordering of stripe domains due to the modified electrostatic boundary conditions in BFO/LSCO/NGO film. Additional studies on B-site doping of Nb ions in BFO films showed change in the domain structures due to doping induced change in lattice anisotropy while maintaining the stripe domain morphology with 109° domain wall. This long-range array of ferroelectric and ferroelastic domains can be useful for optoelectronic devices and ferroelastic templates for strain coupled artificial magnetoelectric heterostructures.

We studied switchable photovoltaic and photo-diode characteristics of Pt/(Bi0.9Sm0.1)(Fe0.97Hf0.03)O3/ LaNiO3 (Pt/BSFHO/LNO) heterostructures integrated on Si (100). The directions of photocurrent (JSC) and rectification are found to be reversibly switchable after applying external poling voltages. In pristine state, metal-ferroelectric-metal capacitor Pt/BSFHO/LNO shows JSC ∼ 32 μA/cm2 and VOC ∼ 0.04 V, which increase to maximum value of JSC ∼ 303 (−206) μA/cm2 and VOC ∼ −0.32 (0.26) V after upward (downward) poling at ±8 V. We believe that Schottky barrier modulation by polarization flipping at Pt/BSFHO interface could be a main driving force behind switchable photovoltaic and rectifying diode characteristics of Pt/BSFHO/LNO heterostructures.

We are reporting on the improved ferroelectric and photovoltaic properties of (Nd3+, V5+) co-doped (Bi0.95Nd0.05)(Fe1−x V x )O3 (BNFVO) (x = 0.01, 0.03) thin films grown by PLD. BNFVO thin films showed reduced leakage current, lower optical bandgap, and improved ferroelectricity compared to BFO films, which can be explained by valance change and extinguishing oxygen vacancies due to the doping effect. Piezoresponse force microscopy measurements showed an improved domain back switching in doped thin films indicating that the suppression of oxygen vacancies offset the effect of polarization flipping caused by doping. Further, we found a relatively stable and enhanced photovoltaic effect in BNFVO films with an order of magnitude higher photocurrent and almost doubled photovoltage in comparison to BFO films, which can be explained by less recombination between hopping electrons and oxygen vacancies. Our results demonstrate the significance of dopant selection to suppress the oxygen vacancies for improved ferroelectric and photovoltaic properties of BFO films.

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Graphitic carbon nanospheres (GCNSs) were prepared by a unique acidic treatment of multi-walled nanotubes. Spherical morphology with a narrow size distribution was confirmed by transmission electron microscopy studies. The room temperature Raman spectra showed a clear signature of D- and G-peaks at around 1350 and 1591 cm−1, respectively. Temperature dependent Raman scattering measurements were performed to understand the phonon dynamics and first order temperature coefficients related to the D- and G-peaks. The temperature dependent Raman spectra in a range of 83–473 K were analysed, where the D-peak was observed to show a red-shift with increasing temperature. The relative intensity ratio of D- to G-peaks also showed a significant rise with increasing temperature. Such a temperature dependent behaviour can be attributed to lengthening of the C-C bond due to thermal expansion in material. The estimated value of the thermal conductivity of GCNSs ∼0.97 W m−1 K−1 was calculated using Raman spectroscopy. In ...

Dye sensitized solar cells (DSSCs) have engendered great research interest with promising potentials in solar cell technology for their high power conversion efficiencies. In this study, we report a plasmon enhanced reduced graphene oxide (RGO)-TiO2 nanocomposite electron injection layer towards the improved performance of DSSCs. Graphene oxide was synthesized using modified Hummer's method and further treated with hydrazine hydrate to obtain reduced graphene oxide (RGO). Structural and morphological properties of as synthesized RGO, RGO- TiO2 and RGO-TiO2-Au nanopaticle (Au NP) composites were investigated using Raman spectroscopy and ultra-high resolution transmission electron microscope (UHRTEM). Raman mapping was performed to further characterize the composite photoanodes.. Photovoltaic performance of DSSC containing composite photoanodes were studied, on keeping the RGO concentration constant and varying the Au NP concentration. A systematic increment in the short circuit current density was noticed while the open circuit voltage (VOC ~ 0.72 V) of DSSC was observed to be constant by increasing Au NP concentration up to an optimal value, probably due to the combined effect of slight rise in quasi-Fermi level and higher carrier transport rate in the resultant composite. Thus, a significant enhancement in the overall photovoltaic performance was observed in DSSCs with RGO- TiO2-Au NP composite photoanode.