Lieber. 2012. Tuning light absorption in core/shell silicon nanowire photovoltaic devices through morphological design. Nano Letters 12(9): 4971-4976.Published Version

Here, we demonstrate that ultrafast two dimensional infrared (2D-IR) spectroscopy provides evidence for the transition state involved in a simple thermal chemical reaction, the fluxionality of Fe(CO) 5 .Fluxionality refers to the rearrangement of a molecule between chemically indistinguishable structures.These reactions produce no net change in molecular structure, yet they are important for understanding the basic chemical behavior and reactivity of molecules in solution.Department of Chemistry, University of California, Berkeley, California 94720, and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 * These authors contributed equally and are listed alphabetically † corresponding author email address: cbharris@berkeley.edu 2 Fe(CO) 5 , an organometallic complex with five CO ligands arranged in a trigonal bipyramidal geometry, is a textbook example for fluxionality. In the late 1950s, nuclear magnetic resonance (NMR) spectroscopy revealed that this molecule rapidly exchanges its CO ligands between axial and equatorial sites. The 13 C NMR spectrum of 13 C labeled Fe(CO) 5 exhibits only a single peak at all accessibly measured solution temperatures, indicating that 13 C nuclei shift between axial and equatorial positions faster than NMR spectroscopy is able to distinguish these two chemical environments (2)(3)(4). Careful analysis of IR, Raman, and NMR spectra of Fe(CO) 5 and various derivatives suggests that the exchange process possesses a low barrier and occurs on a time-scale of picoseconds (4)(5)(6). Nevertheless, the dynamics in solution have not been quantified.From a general perspective, fluxional processes are simple chemical reactions in which a molecule briefly rearranges to a new symmetry and geometry as it crosses a transition state and then returns to its original geometry as it completes the reaction. We show for Fe(CO) 5 that during this process energy is exchanged between the vibrational modes of the reactive ligands. Quantification of this energy exchange provides direct information on the time-scale, transition state, and consequently, mechanism of the reaction.2D-IR spectroscopy has recently received much attention for its ability to monitor thermal reactions and chemical exchange on the femtosecond and picosecond time-scales. Conventionally, ultrafast timing of chemical reactions is achieved by photoinitiating the reactions with a short, intense laser pulse which electronically excites the molecules and typically leaves them with significant excess energy (1). In comparison, 2D-IR spectroscopy only requires vibrational excitation with an ultrafast IR laser pulse and allows the investigation of an entirely different class of thermally activated reactions in liquids at or close to equilibrium. Although similar to one-dimensional IR-pump, IR-probe experiments (1D-IR), 2D-IR spectroscopy separates the contributions to the 1D-IR spectrum into two frequency dimensions, which provides information on the correlations, anharmonicities, and exchange dynamics...

Silicon nanowires (NWs) could enable low-cost and efficient photovoltaics, though their performance has been limited by nonideal electrical characteristics and an inability to tune absorption properties. We overcome these limitations through controlled synthesis of a series of polymorphic core/multishell NWs with highly crystalline, hexagonally-faceted shells, and well-defined coaxial p-type∕ n-type (p∕n) and p∕intrinsic∕n (p∕i∕n) diode junctions. Designed 200-300 nm diameter p∕i∕n NW diodes exhibit ultralow leakage currents of approximately 1 fA, and open-circuit voltages and fillfactors up to 0.5 V and 73%, respectively, under one-sun illumination. Single-NW wavelength-dependent photocurrent measurements reveal size-tunable optical resonances, external quantum efficiencies greater than unity, and current densities double those for silicon films of comparable thickness. In addition, finite-difference-time-domain simulations for the measured NW structures agree quantitatively with the photocurrent measurements, and demonstrate that the optical resonances are due to Fabry-Perot and whispering-gallery cavity modes supported in the high-quality faceted nanostructures. Synthetically optimized NW devices achieve current densities of 17 mA∕cm 2 and power-conversion efficiencies of 6%. Horizontal integration of multiple NWs demonstrates linear scaling of the absolute photocurrent with number of NWs, as well as retention of the high open-circuit voltages and short-circuit current densities measured for single NW devices. Notably, assembly of 2 NW elements into vertical stacks yields short-circuit current densities of 25 mA∕cm 2 with a backside reflector, and simulations further show that such stacking represents an attractive approach for further enhancing performance with projected efficiencies of >15% for 1.2 μm thick 5 NW stacks.nanodevices | nanomaterials | nanophotonics | optical nanocavities | solar cells N anostructures and nanostructured materials may enable next-generation solar cells by providing for efficient charge separation (1-16) and tunable optical absorption (11,(17)(18)(19). Semiconductor nanowires (NWs) have exhibited promising efficiencies as single NW photovoltaic elements (8)(9)(10)(11)(20)(21)(22) and as vertical arrays configured as photovoltaic (23-26) and photoelectrochemical (27, 28) solar cells, where the vertical array has been used to enhance light absorption (29). In the case of Si-based nanostructures, where Si photovoltaics represent benchmark systems with attractive material abundance and cost (30), the efficiency of NW devices has typically been limited by poor electrical performance and lack of tunable control of absorption properties at specific and broadband wavelengths. For example, previous reports of coaxial (8) and axially modulated (9) p-i-n single-NW photovoltaic devices yielded open-circuit voltages (V OC ) below 0.29 V, and, for coaxial devices, large leakage currents >1 pA. Furthermore, to accurately identify potentially unique absorption modes through photocurrent spectra ...