@article{milleville_chen_lennon_cleveland_kumar_zhang_bork_tessier_lebeau_chase_et al._2019, title={Engineering Efficient Photon Upconversion in Semiconductor Heterostructures}, volume={13}, ISSN={["1936-086X"]}, DOI={10.1021/acsnano.8b07062}, abstractNote={Photon upconversion is a photophysical process in which two low-energy photons are converted into one high-energy photon. Photon upconversion has broad appeal for a range of applications from biomedical imaging and targeted drug release to solar energy harvesting. Current upconversion nanosystems, including lanthanide-doped nanocrystals and triplet-triplet annihilation molecules, have achieved upconversion quantum yields on the order of 10-30%. However, the performance of these materials is hampered by inherently narrow absorption cross sections and fixed energy levels originating in atomic, ionic, or molecular states. Semiconductors, on the other hand, have inherently wide absorption cross sections. Moreover, recent advances enable the synthesis of colloidal semiconductor nanoparticles with complex heterostructures that can control band alignments and tune optical properties. We synthesize and characterize a three-component heterostructure that successfully upconverts photons under continuous-wave illumination and solar-relevant photon fluxes. The heterostructure is composed of two cadmium selenide quantum dots (QDs), an absorber and emitter, spatially separated by a cadmium sulfide nanorod (NR). We demonstrate that the principles of semiconductor heterostructure engineering can be applied to engineer improved upconversion efficiency. We first eliminate electron trap states near the surface of the absorbing QD and then tailor the band gap of the NR such that charge carriers are funneled to the emitting QD. When combined, these two changes result in a 100-fold improvement in photon upconversion performance.}, number={1}, journal={ACS NANO}, author={Milleville, Christopher C. and Chen, Eric Y. and Lennon, Kyle R. and Cleveland, Jill M. and Kumar, Abinash and Zhang, Jing and Bork, James A. and Tessier, Ansel and LeBeau, James M. and Chase, D. Bruce and et al.}, year={2019}, month={Jan}, pages={489–497} }
 @article{mcginnis_thomson_davis_chen_michel_lamb_2001, title={In situ cleaning of GaN/6H-SiC substrates in NH3}, volume={222}, ISSN={["0022-0248"]}, DOI={10.1016/s0022-0248(00)00947-7}, abstractNote={Metalorganic chemical vapor deposition-grown GaN on 6H-SiC substrates were cleaned by annealing in an NH3 flux. Oxygen contamination was removed by thermal desorption, and carbon removal was facilitated by reaction with NH3. The GaN(0 0 0 1) surface after NH3 beam cleaning at 730°C was smooth with distinct atomic steps. The roughness (0.20 nm RMS) was only slightly greater than that of the untreated substrate (0.17 nm RMS). Carbon and oxygen concentrations were reduced to background levels (∼1 at%) by annealing in an NH3 flux at 800°C. The surface step structure was destroyed by annealing in an NH3 flux of 4×1015 cm−2 s−1 from a seeded supersonic beam; however, annealing in an NH3 flux of 7×1015 cm−2 s−1 from a leak valve inhibited surface roughening and produced a relatively smooth surface (0.28 nm RMS) with a 3×3 R30° reconstruction. We infer from the effects of annealing temperature and NH3 flux that the observed surface roughening is due to GaN decomposition.}, number={3}, journal={JOURNAL OF CRYSTAL GROWTH}, author={McGinnis, AJ and Thomson, D and Davis, RF and Chen, E and Michel, A and Lamb, HH}, year={2001}, month={Jan}, pages={452–458} }