Thomas E. Furtak

Athermal photofluidization of glasses.

Azobenzene and its derivatives are among the most important organic photonic materials, with their photo-induced trans-cis isomerization leading to applications ranging from holographic data storage and photoalignment to photoactuation and nanorobotics. A key element and enduring mystery in the photophysics of azobenzenes, central to all such applications, is athermal photofluidization: illumination that produces only a sub-Kelvin increase in average temperature can reduce, by many orders of magnitude, the viscosity of an organic glassy host at temperatures more than 100u2009K below its thermal glass transition. Here we analyse the relaxation dynamics of a dense monolayer glass of azobenzene-based molecules to obtain a measurement of the transient local effective temperature at which a photo-isomerizing molecule attacks its orientationally confining barriers. This high temperature (T(loc)~800u2009K) leads directly to photofluidization, as each absorbed photon generates an event in which a local glass transition temperature is exceeded, enabling collective confining barriers to be attacked with near 100% quantum efficiency.

Growth of diamond films on stainless steel

Abstract Continuous diamond films have been deposited on 304 stainless steel with a silicon intermediate layer by means of hot-filament chemical vapor deposition. The most serious problems for growing diamond films on ferrous metals are a long nucleation period, the catalytic effect of iron, and thermal expansion mismatch. In order to reduce these unwanted effects, three approaches have been adopted. First, an intermediate layer was used to block carbon diffusion, enhance adhesion and suppress sp 2 carbon formation. Second, oxygen-assisted low temperature deposition was used to minimized the thermal expansion effect and also avoid the phase transition of iron alloys. Third, initial and secondary nucleation was enhanced to yield small grain sizes in the continuous film. The morphology and quality of the deposited films were characterized by scanning electron microscopy and Raman spectroscopy respectively. Pull-off adhesion tests showed that the intermediate layer was strongly bonded to both the steel substrate and diamond film.

Spiral plasmonic nanoantennas as circular polarization transmission filters

We present simulation and experimental results for easily fabricated spiral plasmonic antenna analogues providing circular polarization selectivity. One circular polarization state is concentrated and transmitted through a subwavelength aperture, while the opposite circular state is blocked. The spectral bandwidth, efficiency, and extinction ratios are tunable through geometric parameters. Integration of such structures onto a focal plane array in conjunction with linear micropolarizers enables complete Stokes vector imaging, that, until now, has been difficult to achieve. An array of these structures forms a plasmonic metamaterial that exhibits high circular dichroism.

Size-Dependent Exciton Formation Dynamics in Colloidal Silicon Quantum Dots.

We report size-dependent exciton formation dynamics within colloidal silicon quantum dots (Si QDs) using time-resolved terahertz (THz) spectroscopy measurements. THz photoconductivity measurements are used to distinguish the initially created hot carriers from excitons that form at later times. At early pump/probe delays, the exciton formation dynamics are revealed by the temporal evolution of the THz transmission. We find an increase in the exciton formation time, from ∼500 to ∼900 fs, as the Si QD diameter is reduced from 7.3 to 3.4 nm and all sizes exhibit slower hot-carrier relaxation times compared to bulk Si. In addition, we determine the THz absorption cross section at early delay times is proportional to the carrier mobility while at later delays is proportional to the exciton polarizability, αX. We extract a size-dependent αX and find an ∼r(4) dependence, consistent with previous reports for quantum-confined excitons in CdSe, InAs, and PbSe QDs. The observed slowing in exciton formation time for smaller Si QDs is attributed to decreased electron-phonon coupling due to increased quantum confinement. These results experimentally verify the modification of hot-carrier relaxation rates by quantum confinement in Si QDs, which likely plays a significant role in the high carrier multiplication efficiency observed in these nanomaterials.

Nickel oxide interlayer films from nickel formate–ethylenediamine precursor: influence of annealing on thin film properties and photovoltaic device performance

An organometallic ink based on the nickel formate–ethylenediamine (Ni(O2CH)2(en)2) complex forms high performance NiOx thin film hole transport layers (HTL) in organic photovoltaic (OPV) devices. Improved understanding of these HTLs functionality can be gained from temperature-dependent decomposition/oxidation chemistries during film formation and corresponding chemical structure-function relationships for energetics, charge selectivity, and transport in photovoltaic platforms. Investigations of as-cast films annealed in air (at 150 °C–350 °C), with and without subsequent O2-plasma treatment, were performed using thermogravimetric analysis, Fourier transform infrared spectroscopy, ultraviolet and X-ray photoelectron spectroscopy, and spectroscopic ellipsometry to elucidate the decomposition and oxidation of the complex to NiOx. Regardless of the anneal temperature, after exposure to O2-plasma, these HTLs exhibit work functions greater than the ionization potential of a prototype donor polymer poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT), thereby meeting a primary requirement of energy level alignment. Thus, bulk-heterojunction (BHJ), OPV solar cells made on this series of NiOx HTLs all exhibit similar open circuit voltages (Voc). In contrast, the short circuit currents increase significantly from 1.7 to 11.2 mA cm−2 upon increasing the anneal temperature from 150 °C to 250 °C. Concomitantly, increased conductivity and electrical homogeneity of NiOx thin films are observed at the nanoscale using conductive tip-AFM. Similar Voc observed for all the O2-plasma treated NiOx interlayers and variations to nanoscale conductivity suggest that the HTLs all form charge selective contacts and that their carrier extraction efficiency is determined by the amount of precursor conversion to NiOx. The separation of these two properties: selectivity and conductivity, sheds further light on charge selective interlayer functionality.

Conjugated Phosphonic Acid Modified Zinc Oxide Electron Transport Layers for Improved Performance in Organic Solar Cells

Phosphonic acid modification of zinc oxide (ZnO) electron transport layers in inverted P3HT:ICBA solar cells was studied to determine the effect of conjugated linkages between the aromatic and phosphonic acid attachment groups. For example, zinc oxide treated with 2,6-difluorophenylvinylphosphonic acid, having a conjugated vinyl group connecting the aromatic moiety to the phosphonic acid group, showed a 0.78 eV decrease in the effective work function versus unmodified ZnO, whereas nonconjugated 2,6-difluorophenylethylphosphonic acid resulted in a 0.57 eV decrease, as measured by Kelvin probe. This resulted in an average power conversion efficiency of 5.89% for conjugated 2,6-difluorophenyvinylphosphonic acid modified solar cells, an improvement over unmodified (5.24%) and nonconjugated phosphonic acid modified devices (5.64%), indicating the importance of the conjugated linkage.

Ultra-high extinction ratio micropolarizers using plasmonic lenses

The design of a new type of plasmonic ultra-high extinction ratio micropolarizing transmission filter is presented along with an experimental demonstration. A pair of dielectric coated metal gratings couple incident TM polarized light into surface plasmons, which are fed into a central metal-insulator-metal (MIM) waveguide, followed by transmission through a sub-wavelength aperture. Extinction ratios exceeding 10¹¹ are predicted by finite element simulation. Good absolute agreement for both the spectral and polarization response is obtained between measurement and simulations using measured geometric parameters. The filters can be easily fabricated and sized to match the pixel pitch of current focal plane arrays.

Raman characterization of epitaxial Cu–In–Se thin films

Abstract We have fabricated epitaxial chalcopyrite alloys with varying compositions and thicknesses on GaAs (001) substrates using a simple physical vapor deposition method. Growth of the films was characterized with Raman spectroscopy as well as by other analytical techniques. We identified both CuInSe 2 (001) and CuIn 3 Se 5 (001). All of the spectra were dominated by the A 1 (Γ 1 (1) [W 1 ]) non-polar optical mode at 172 cm −1 for CuInSe 2 and 152 cm −1 for the CuIn 3 Se 5 phase. In addition, Raman spectra for the thinner layers indicated that these films are under compressive stress due to the lattice mismatch between the films and the substrate.

Ultrafast Electrical Measurements of Isolated Silicon Nanowires and Nanocrystals.

We simultaneously determined the charge carrier mobility and picosecond to nanosecond carrier dynamics of isolated silicon nanowires (Si NWs) and nanocrystals (Si NCs) using time-resolved terahertz spectroscopy. We then compared these results to data measured on bulk c-Si as a function of excitation fluence. We find >1 ns carrier lifetimes in Si NWs that are dominated by surface recombination with surface recombination velocities (SRV) between ∼1100-1700 cm s(-1) depending on process conditions. The Si NCs have markedly different decay dynamics. Initially, free-carriers are produced, but relax within ∼1.5 ps to form bound excitons. Subsequently, the excitons decay with lifetimes >7 ns, similar to free carriers produced in bulk Si. The isolated Si NWs exhibit bulk-like mobilities that decrease with increasing excitation density, while the hot-carrier mobilities in the Si NCs are lower than bulk mobilities and could only be measured within the initial 1.5 ps decay. We discuss the implications of our measurements on the utilization of Si NWs and NCs in macroscopic optoelectronic applications.

Tuning zinc oxide/organic energy level alignment using mixed triethoxysilane monolayers

Interfacial energy level alignment influences several critical organic optoelectronic device characteristics including charge transfer barriers, turn-on voltage, and open circuit voltage (Voc). Introduction of dipolar molecular monolayers on metal oxide surfaces has allowed improvements in device performance as well as fundamental studies of energy level alignment in these devices. We demonstrate that dipolar mixed monolayers can be covalently bonded to zinc oxide (ZnO) through the triethoxysilane chemical attachment scheme, and that these monolayers tune the work function of the ZnO surface over 0.6 eV. We then employ mixed monolayer-treated ZnO surfaces as the electron-selective contact in inverted bulk heterojunction photovoltaic devices to determine the correlation between the Voc and the work function of the contact. We find the relationship of −0.14 V eV−1 between the Voc and contact work function is consistent with current results and theories of contact influence on Voc.