Surface third-harmonic generation at a two-photon-polymerized micro-interferometer for real-time on-chip refractive index monitoring.

A micro-interferometer based on surface third-harmonic generation (THG) at two-photon-polymerized SU-8 cuboids for real-time monitoring of the refractive index changes of target fluids, which can be easily integrated into microfluidic photonic systems, is demonstrated. The third-harmonic (TH) interferogram is selectively generated only from the target volume by a simple vertical pumping, thereby eliminating the needs for complicated coupling and alignments. The dependence of the generated TH to the input pump polarization state is thoroughly investigated. The THG efficiency by linearly polarized excitation is found to be 2.6 × 10-7, which is the most efficient at the SU-8-air interface and independent of the input polarization direction. The THG efficiency from the SU-8-air interface is 12.17 times higher than that from the glass-air interface and 4.93 times higher than that from the SU-8-glass interface. Real-time monitoring of argon gas pressure is demonstrated using the micro- interferometer. The surface TH from two-photon-polymerized 3D structures offers novel design flexibility to the nonlinear optical light sources for microfluidic and microelectronic devices.

Generating free charges by carrier multiplication in quantum dots for highly efficient photovoltaics.

CONSPECTUS: In a conventional photovoltaic device (solar cell or photodiode) photons are absorbed in a bulk semiconductor layer, leading to excitation of an electron from a valence band to a conduction band. Directly after photoexcitation, the hole in the valence band and the electron in the conduction band have excess energy given by the difference between the photon energy and the semiconductor band gap. In a bulk semiconductor, the initially hot charges rapidly lose their excess energy as heat. This heat loss is the main reason that the theoretical efficiency of a conventional solar cell is limited to the Shockley-Queisser limit of ∼33%. The efficiency of a photovoltaic device can be increased if the excess energy is utilized to excite additional electrons across the band gap. A sufficiently hot charge can produce an electron-hole pair by Coulomb scattering on a valence electron. This process of carrier multiplication (CM) leads to formation of two or more electron-hole pairs for the absorption of one photon. In bulk semiconductors such as silicon, the energetic threshold for CM is too high to be of practical use. However, CM in nanometer sized semiconductor quantum dots (QDs) offers prospects for exploitation in photovoltaics. CM leads to formation of two or more electron-hole pairs that are initially in close proximity. For photovoltaic applications, these charges must escape from recombination. This Account outlines our recent progress in the generation of free mobile charges that result from CM in QDs. Studies of charge carrier photogeneration and mobility were carried out using (ultrafast) time-resolved laser techniques with optical or ac conductivity detection. We found that charges can be extracted from photoexcited PbS QDs by bringing them into contact with organic electron and hole accepting materials. However, charge localization on the QD produces a strong Coulomb attraction to its counter charge in the organic material. This limits the production of free charges that can contribute to the photocurrent in a device. We show that free mobile charges can be efficiently produced via CM in solids of strongly coupled PbSe QDs. Strong electronic coupling between the QDs resulted in a charge carrier mobility of the order of 1 cm(2) V(-1) s(-1). This mobility is sufficiently high so that virtually all electron-hole pairs escape from recombination. The impact of temperature on the CM efficiency in PbSe QD solids was also studied. We inferred that temperature has no observable effect on the rate of cooling of hot charges nor on the CM rate. We conclude that exploitation of CM requires that charges have sufficiently high mobility to escape from recombination. The contribution of CM to the efficiency of photovoltaic devices can be further enhanced by an increase of the CM efficiency above the energetic threshold of twice the band gap. For large-scale applications in photovoltaic devices, it is important to develop abundant and nontoxic materials that exhibit efficient CM.

Epitaxially connected PbSe quantum-dot films: controlled neck formation and optoelectronic properties.

Ligand exchange is a much-used method to increase the conductivity of colloidal quantum-dot films by replacing long insulating ligands on quantum-dot surfaces with shorter ones. Here we show that while some ligands indeed replace the original ones as expected, others may be used to controllably remove the native ligands and induce epitaxial necking of specific crystal facets. In particular, we demonstrate that amines strip lead oleate from the (100) surfaces of PbSe quantum dots. This leads to necking of QDs and results in cubic superlattices of epitaxially connected QDs. The number of amine head-groups as well as the carbon chain length of linear diamines is shown to control the extent of necking. DFT calculations show that removal of Pb(oleate)2 from (100) surfaces is exothermic for all amines, but the driving force increases as monoamines < long diamines < short diamines < tetramines. The neck formation and cubic ordering results in a higher optical absorption cross section and higher charge carrier mobilities, thereby showing that the use of the proper multidentate amine molecules is a powerful tool to create supercrystals of epitaxially connected PbSe QDs with controlled electronic coupling.

High charge-carrier mobility enables exploitation of carrier multiplication in quantum-dot films.

Carrier multiplication, the generation of multiple electron-hole pairs by a single photon, is of great interest for solar cells as it may enhance their photocurrent. This process has been shown to occur efficiently in colloidal quantum dots, however, harvesting of the generated multiple charges has proved difficult. Here we show that by tuning the charge-carrier mobility in quantum-dot films, carrier multiplication can be optimized and may show an efficiency as high as in colloidal dispersion. Our results are explained quantitatively by the competition between dissociation of multiple electron-hole pairs and Auger recombination. Above a mobility of ~1 cm(2) V(-1) s(-1), all charges escape Auger recombination and are quantitatively converted to free charges, offering the prospect of cheap quantum-dot solar cells with efficiencies in excess of the Shockley-Queisser limit. In addition, we show that the threshold energy for carrier multiplication is reduced to twice the band gap of the quantum dots.

Photoconductivity of PbSe quantum-dot solids: dependence on ligand anchor group and length.

The assembly of quantum dots is an essential step toward many of their potential applications. To form conductive solids from colloidal quantum dots, ligand exchange is required. Here we study the influence of ligand replacement on the photoconductivity of PbSe quantum-dot solids, using the time-resolved microwave conductivity technique. Bifunctional replacing ligands with amine, thiol, or carboxylic acid anchor groups of various lengths are used to assemble quantum solids via a layer-by-layer dip-coating method. We find that when the ligand lengths are the same, the charge carrier mobility is higher in quantum-dot solids with amine ligands, while in quantum-dot solids with thiol ligands the charge carrier lifetime is longer. If the anchor group is the same, the charge carrier mobility is ligand length dependent. The results show that the diffusion length of charge carriers can reach several hundred nanometers.

Mixing does the magic: a rapid synthesis of high surface area noble metal nanosponges showing broadband nonlinear optical response.

Here we report an instantaneous formation of high surface area metal nanosponges through a one-step inexpensive method in a completely green solvent, water. Merely by optimizing the concentration of the precursors and the reducing agent, we were able to generate a three-dimensional porous structure made up of nanowire networks. This is a general process, involves a simple, room temperature reduction of metal salts with sodium borohydride, and is therefore scalable to any amount. Further, these nanoporous metals because of their network structures show optical limiting behavior of a true broadband nature that would find applications in optoelectronic nanodevices.