David L. Patrick

Zero-Reabsorption Doped-Nanocrystal Luminescent Solar Concentrators

Optical concentration can lower the cost of solar energy conversion by reducing photovoltaic cell area and increasing photovoltaic efficiency. Luminescent solar concentrators offer an attractive approach to combined spectral and spatial concentration of both specular and diffuse light without tracking, but they have been plagued by luminophore self-absorption losses when employed on practical size scales. Here, we introduce doped semiconductor nanocrystals as a new class of phosphors for use in luminescent solar concentrators. In proof-of-concept experiments, visibly transparent, ultraviolet-selective luminescent solar concentrators have been prepared using colloidal Mn(2+)-doped ZnSe nanocrystals that show no luminescence reabsorption. Optical quantum efficiencies of 37% are measured, yielding a maximum projected energy concentration of ∼6× and flux gain for a-Si photovoltaics of 15.6 in the large-area limit, for the first time bounded not by luminophore self-absorption but by the transparency of the waveguide itself. Future directions in the use of colloidal doped nanocrystals as robust, processable spectrum-shifting phosphors for luminescent solar concentration on the large scales required for practical application of this technology are discussed.

Simulations of luminescent solar concentrators: Effects of polarization and fluorophore alignment

We model the effects of dye molecule alignment on the collection efficiency of luminescent solar concentrators (LSCs). A Monte Carlo model for photon transport in LSC’s is derived and utilized, which incorporates the effects of fluorescent-dye-molecular alignment and the subsequent control over absorption, emission, and propagation properties. We focus on the effects of molecular alignment statistics on photon absorption and subsequent emission, including polarization and propagation direction imparted by dipole direction, to model device light-capture efficiency, defined as the ratio of the amount of light reaching particular slab edges to that incident on a face. We find that modest control of alignment, coupled with reasonable and attainable emission-absorption dipole angles, can produce very large collection efficiencies for a range of device parameters. We note that efficiencies for small values of dye molecule Stoke’s shift may be made as large as those for homogeneous (unaligned) systems with large...

Atomistic simulations of fluid structure and solvation forces in atomic force microscopy

Abstract We describe results of atomistic molecular dynamics simulations modelling an atomic force microscope (AFM) tip immersed in a fluid. Both the tip and the surface are modelled by rigid arrays of atoms. The tip is pyramidal and the surface is the (100) face of a fcc crystal. The focus is on the solvation forces acting on the tip and on the surface and their relation to the structural and dynamic properties of the fluid. Fluid particles in the neighborhood of the tip-surface junction are found to be highly ordered compared to the bulk, as shown by localized variations in the average fluid density. The atomistic nature of the model gives rise to several effects related to the discrete sizes of the fluid, tip, and surface particles which are not observed in continuum-based theories. A number of simulated force-distance curves are presented, along with an analysis of the effect of changing fluid particle size, tip (lateral) position, tip shape, and the lyocompatability of the tip and surface materials. The atomic-scale distribution of fluid-surface forces is examined for various positions of the tip, and the extent to which the fluid can act as a “cushion” by increasing the effective area of the tip-surface interaction is studied. The effect of a fluid on AFM imaging is investigated by generating “fluid images”, which are shown to be comparable in magnitude to the direct tip-surface interaction in the noncontact mode. We compare images generated by defective and defect-free surfaces, and analyse the fluid-tip forces acting in a lateral direction. An image formed from fluid forces acting in the direction of the surface normal does not show the presence of a vacancy, but an image formed from lateral fluid forces does.

Unusual aspects of superperiodic features on highly oriented pyrolytic graphite

Scanning tunneling microscopy was used to study several types of superstructures on highly oriented pyrolytic graphite surfaces. The superlattices are unlike any reported to date and cannot be explained by the moire-rotation hypothesis. Calculations using a simple model to test the moire pattern hypothesis with computer-generated graphite layers indicate that only very unreasonable ratios for the relative influences of the second and third layers of graphite produce a superlattice comparable to an observed superstructure. These findings show that the current moire-rotation model is insufficient to account for the range of observed superlattice phenomena on graphite.

Comprehensive Analysis of Escape-Cone Losses from Luminescent Waveguides

Luminescent waveguides (LWs) occur in a wide range of applications, from solar concentrators to doped fiber amplifiers. Here we report a comprehensive analysis of escape-cone losses in LWs, which are losses associated with internal rays making an angle less than the critical angle with a waveguide surface. For applications such as luminescent solar concentrators, escape-cone losses often dominate all others. A statistical treatment of escape-cone losses is given accounting for photoselection, photon polarization, and the Fresnel relations, and the model is used to analyze light absorption and propagation in waveguides with isotropic and orientationally aligned luminophores. The results are then compared to experimental measurements performed on a fluorescent dye-doped poly(methyl methacrylate) waveguide.

Ultrahigh vacuum surface science chamber with integral scanning tunneling microscope

The construction of an ultrahigh vacuum (UHV) surface science chamber equipped with the standard surface analytical techniques and a connected companion UHV chamber containing a scanning tunneling microscope (STM) has been completed. The novel aspects of this experimental system are: the combination of many spatially averaging techniques with STM; a sample holder which is capable of in situ transfer between these various capabilities; variable temperature operation; in situ tip‐sample approach without mechanical feedthroughs; and various novel software aspects. The sample transfer mechanism allows the sample to be transferred onto the main manipulator and heated or cooled with thermocouple monitoring while electrical isolation from the chamber ground is maintained. The sample then can be transferred in vacuo to the UHV STM for further study. STM tips can be transferred into and out of vacuum and positioned for sputtering and UHV analysis. The various design details which allow for in vacuo transfers will ...

Getting organized at the nanoscale with thermotropic liquid crystal solvents

This paper summarizes recent progress toward an emerging, unconventional application of thermotropic liquid crystals (LCs) - their use as solvents for controlling the assembly of non-LC nanoscale building blocks. LCs offer a number of potential advantages compared to conventional isotropic solvents, including the ability to influence building block orientation and other structural properties. Strategies are reviewed for the exploitation of LC media to engineer order in a range of systems, including oriented organic monolayers, chiral films, and nanometer-scale particles.

Organic-Vapor-Liquid-Solid Deposition with an Impinging Gas Jet

A method for rapid, mass-efficient deposition of highly crystalline organic films under near ambient conditions of pressure and temperature is reported based on delivery of an organic precursor via an impinging gas jet to a substrate coated by a thin liquid solvent layer. Films of the organic semiconductor tetracene were deposited by sublimation into a flow of argon carrier gas directed at an indium-tin-oxide/glass substrate coated by a thin layer of bis(2-ethylhexyl)sebecate, and growth was followed in situ with optical microscopy. A fluid dynamics model is applied to account for the gas phase transport and aggregation, and the results compared to experiment. The combination of gas jet delivery with an organic-vapor-liquid-solid growth mechanism leads to larger crystals and lower nucleation densities than on bare surfaces, with markedly different nucleation and growth kinetics. An explanation based on enhanced solution-phase diffusivity and a larger critical nucleus size in the liquid layer is proposed t...

Molecular dynamics simulation of atomic force microscopy: imaging single-atom vacancies on Ag(001) and Pt(001)

In a growing number of examples, the atomic force microscope (AFM) has demonstrated an ability to provide genuine atomic-resolution images of single crystal surfaces by resolving atomic-sized surface features. However, to date, all such examples have either used ionic or covalent crystal samples; metals are notably absent from the list of materials yielding true atomic resolution. This study has used molecular dynamics simulation to study the interaction of a single-atom Ag tip with Ag(001) and Pt(001) surfaces in order to address the question of whether or not atomic-resolution AFM imaging of metal surfaces is likely to be feasible, and under what conditions. Using a constant force imaging procedure, a narrow window of imaging forces between 1 and 3 nN (attractive) was identified in which genuine atomic resolution was achieved without tip-induced disruption of the surface structure. Forces outside this range caused rearrangement of surface atoms or penetration of the tip into the surface. These findings suggest that severe experimental conditions are required to image metal surfaces with AFM at atomic resolution.

Attenuated total reflection Fourier-transform infrared spectroscopic study of ion–solvent and ion–ion interactions in lithium perchlorate–nitromethane solutions

Lithium perchlorate solutions in nitromethane, 0.04–0.18 mol dm–3, have been investigated using attenuated total reflection Fourier-transform infrared (ATR FTIR) spectroscopy. Despite the relatively poor electron-donating ability of nitromethane, some evidence for lithium ion coordination is seen. Frequency shifts for the symmetric NO2 stretching fundamental of +7 cm–1, and for the symmetric CH3 bending fundamental of +2 cm–1 are found for nitromethane complexed to the alkali-metal cation. The infrared-active absorption for the ClO–4 anion at 1100 cm–1 shows substantial splitting, an effect due to the extensive ion-pairing of the solute species. The distribution and relative concentrations of the different electrolyte species were found to be concentration dependent.