Research Outline

Underlying goals of our research program include (i) synthesis and characterization of novel nanoscale materials, (ii) elucidation of their fundamental optoelectronic properties, and (iii) design and demonstration of functional devices based on these nanoscale materials.

While much of the work in our laboratory has been directed toward solar energy conversion, fundamental research in this expanding field has far reaching implications that extend to other present and future devices that operate on a molecular level. Ultimately, we will be able to rationally design multi-functional nanoscale interfaces to perform specific functions. Our research has also branched out to biomedical imaging, photocatalysis and development of hybrid light-emitting diodes. Summarized below are our recent discoveries that provide new directions for our future research.

Self-Sustained Size Focusing of Colloidal Semiconductor Nanocrystals

Digestive ripening (DR) represents a powerful strategy for improving the size homogeneity of colloidal nanostructures. It relies on the ligand-mediated dissolution of larger nanoparticles in favor of smaller ones and is often considered to be the opposite of Ostwald ripening. Despite its successful application to size-focusing of metal colloids, digestive ripening of semiconductor nanocrystals has received little attention to date. Here, we explore this synthetic niche and demonstrate that ligand-induced ripening of semiconductor nanocrystals exhibits an unusual reaction path. The unique aspect of the DR process in semiconductors lies in the thermally activated particle coalescence, which leads to a significant increase in the nanocrystal size for temperatures above the threshold value (Tth = 200–220 °C). Below this temperature, nanoparticle sizes focus to an ensemble average diameter just like in the case of metal colloids. The existence of the thermal threshold for coalescence offers an expedient strategy for controlling both the particle size and the size dispersion. Such advanced shape control was demonstrated using colloids of CdS, CdSe, CsPbBr3, and CuZnSnS4, where monodisperse samples were obtained across broad diameter ranges. We expect the demonstrated approach to be extended to other semiconductors as a simple strategy for tuning the nanoparticle morphology.

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Razgoniaeva, N.; Yang, M.; Garrett, P.; Kholmicheva, N.; Moroz, P.; Eckard, H.; Royo Romero, L.; Porotnikov, D.; Khon, D.; Zamkov, M. "Just Add Ligands: Self-Sustained Size Focusing of Colloidal Semiconductor Nanocrystals." Chem. Mater., 2018, 30, 1391-1398

One-Dimensional Carrier Confinement in “Giant” CdS/CdSe Excitonic Nanoshells

The emerging generation of quantum dot optoelectronic devices offers an appealing prospect of a size-tunable band gap. The confinement-enabled control over electronic properties, however, requires nanoparticles to be sufficiently small, which leads to a large area of interparticle boundaries in a film. Such interfaces lead to a high density of surface traps which ultimately increase the electrical resistance of a solid. To address this issue, we have developed an inverse energy-gradient core/shell architecture supporting the quantum confinement in nanoparticles larger than the exciton Bohr radius. The assembly of such nanostructures exhibits a relatively low surface-to-volume ratio, which was manifested in this work through the enhanced conductance of solution-processed films. The reported core/shell geometry was realized by growing a narrow gap semiconductor layer (CdSe) on the surface of a wide-gap core material (CdS) promoting the localization of excitons in the shell domain, as was confirmed by ultrafast transient absorption and emission lifetime measurements. The band gap emission of fabricated nanoshells, ranging from 15 to 30 nm in diameter, has revealed a characteristic size-dependent behavior tunable via the shell thickness with associated quantum yields in the 4.4–16.0% range.

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Razgoniaeva, N.; Moroz, P.; Yang, M.; Budkina, D. S.; Eckard, H.; Augspurger, M.; Khon, D.; Tarnovsky, A. N.; Zamkov, M. "One-dimensional carrier confinement in “giant” CdS/CdSe excitonic nanoshells." J. Am. Chem. Soc., 2017, 139, 7815-7822.

Tracking the Energy Flow on Nanoscale via Sample-Transmitted Excitation Photoluminescence Spectroscopy

Tracking the energy flow in nanoscale materials is an important yet challenging goal. Experimental methods for probing the intermolecular energy transfer (ET) are often burdened by the spectral crosstalk between donor and acceptor species, which complicates unraveling their individual contributions. This issue is particularly prominent in inorganic nanoparticles and biological macromolecules featuring broad absorbing profiles. Here, we demonstrate a general spectroscopic strategy for measuring the ET efficiency between nanostructured or molecular dyes exhibiting a significant donor–acceptor spectral overlap. The reported approach is enabled through spectral shaping of the broadband excitation light with solutions of donor molecules, which inhibits the excitation of respective donor species in the sample. The resulting changes in the acceptor emission induced by the spectral modulation of the excitation beam are then used to determine the quantum efficiency and the rate of ET processes between arbitrary fluorophores (molecules, nanoparticles, polymers) with high accuracy. The feasibility of the reported method was demonstrated using a control donor–acceptor system utilizing a protein-bridged Cy3-Cy5 dye pair and subsequently applied for studying the energy flow in a CdSe560-CdSe600 binary nanocrystal film.

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Moroz, P.; Razgoniaeva, N.; He, Y.; Jensen, G.; Eckard, H.; Lu, H. P.; Zamkov, M. "Tracking the Energy Flow on Nanoscale via Sample-Transmitted Excitation Photoluminescence Spectroscopy" ACS Nano, 2017, 11, 4191-4197.

Enhanced Emission of Nanocrystal Solids Featuring Slowly Diffusive Excitons

Solution processing of semiconductor nanocrystal (NC) solids represents an attractive platform for the development of next-generation optoelectronic devices. In search of enhanced light-emitting performance, NC solids are typically designed to have large interparticle gaps that minimize exciton diffusion to dissociative sites. This strategy, however, reduces electrical coupling between nanoparticles in a film, making the injection of charges inefficient. Here, we demonstrate that bright emission from nanocrystal solids can be achieved without compromising their electrical conductivity. Our study shows that solids featuring a low absorption-emission spectral overlap (J) exhibit an intrinsically slower exciton diffusion to recombination centers, promoting longer exciton lifetimes. As a result, enhanced emission is achieved despite a strong electronic coupling. The observed phenomenon was found consistent with a decreased resonant energy transfer in films exhibiting a reduced J value. The inverse correlation between film luminescence and J was revealed through a comparative analysis of CdSe/CdS and ZnSe/CdS solids and further confirmed in two control systems (ZnTe/CdSe and Mn2+-doped ZnCdSe/ZnS). Exceptionally slow exciton diffusion (∼0.3 ms) and high brightness were observed for Mn2+-doped Zn1–xCdxSe/ZnS NC films exhibiting a nearly vanishing J parameter. We expect that the demonstrated combination of electrical coupling and bright emission in nanocrystal solids featuring low J can benefit the development of nanocrystal light-emitting technologies.

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Kholmicheva, N.; Razgoniaeva, N.; Yadav, P.; Lahey, A.; Erickson, C.; Moroz, P.; Gamelin, D.; Zamkov, M. "Enhanced Emission of Nanocrystal Solids Featuring Slowly Diffusive Excitons" J. Phys. Chem. C, 2017, 121, 1477-1487.

Colloidal Synthesis of Monodisperse Semiconductor Nanocrystals through Saturated Ionic Layer Adsorption

We demonstrate a general strategy for the synthesis of colloidal semiconductor nanocrystals (NCs) exhibiting size dispersion below 5%. The present approach relies on the sequential deposition of fully saturated cationic and anionic monolayers onto small-diameter clusters, which leads to focusing of nanocrystal sizes with the increasing particle diameter. Each ionic layer is grown through a room-temperature colloidal atomic layer deposition process that employs a two-solvent mixture to separate the precursor and nanocrystal phases. As a result, unreacted precursors can be removed after each deposition cycle, preventing the secondary nucleation. By using CdS NCs as a model system, we demonstrate that a narrow size dispersion can be achieved through a sequential growth of Cd2+ and S2– layers onto starting CdS cluster “seeds”. Besides shape uniformity, the demonstrated methodology offers an excellent batch-to-batch reproducibility and an improved control over the nanocrystal surface composition. The present synthesis is amenable to other types of semiconductor nanocrystals and can potentially offer a viable alternative to traditional hot-injection strategies of the nanoparticle growth.

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Razgoniaeva, N.; Carrillo, L.; Burchfield, D.; Moroz, P.; Adhikari, P.; Yadav, P.; Khon, D.; Zamkov, M. "Colloidal Synthesis of Monodisperse Semiconductor Nanocrystals through Saturated Ionic Layer Adsorption" Chem. Mater., 2016, 28, 2823-2833.

Plasmonic Nanocrystal Solar Cells Utilizing Strongly Confined Radiation

The ability of metal nanoparticles to concentrate light via the plasmon resonance represents a unique opportunity for funneling the solar energy in photovoltaic devices. The absorption enhancement in plasmonic solar cells is predicted to be particularly prominent when the size of metal features falls below 20 nm, causing the strong confinement of radiation modes. Unfortunately, the ultrashort lifetime of such near-field radiation makes harvesting the plasmon energy in small-diameter nanoparticles a challenging task. Here, we develop plasmonic solar cells that harness the near-field emission of 5 nm Au nanoparticles by transferring the plasmon energy to band gap transitions of PbS semiconductor nanocrystals. The interfaces of Au and PbS domains were designed to support a rapid energy transfer at rates that outpace the thermal dephasing of plasmon modes. We demonstrate that central to the device operation is the inorganic passivation of Au nanoparticles with a wide gap semiconductor, which reduces carrier scattering and simultaneously improves the stability of heat-prone plasmonic films. The contribution of the Au near-field emission toward the charge carrier generation was manifested through the observation of an enhanced short circuit current and improved power conversion efficiency of mixed (Au, PbS) solar cells, as measured relative to PbS-only devices.

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Kholmicheva, N.; Moroz, P.; Rijal, U.; Bastola, E.; Uprety, P.; Liyanage, G.; Razgoniaev, A.; Ostrowski, A.D.; Zamkov, M. "Plasmonic Nanocrystal Solar Cells Utilizing Strongly Confined Radiation" ACS Nano 2014, 8, 12549.

Enhanced Lifetime of Excitons in Nonepitaxial Au/CdS Core/Shell Nanocrystals

The ability of metal nanoparticles to capture light through plasmon excitations offers an opportunity for enhancing the optical absorption of plasmon-coupled semiconductor materials via energy transfer. This process, however, requires that the semiconductor component is electrically insulated to prevent a “backward” charge flow into metal and interfacial states, which causes a premature dissociation of excitons. Here we demonstrate that such an energy exchange can be achieved on the nanoscale by using nonepitaxial Au/CdS core/shell nanocomposites. These materials are fabricated via a multistep cation exchange reaction, which decouples metal and semiconductor phases leading to fewer interfacial defects. Ultrafast transient absorption measurements confirm that the lifetime of excitons in the CdS shell (τ ≈ 300 ps) is much longer than lifetimes of excitons in conventional, reduction-grown Au/CdS heteronanostructures. As a result, the energy of metal nanoparticles can be efficiently utilized by the semiconductor component without undergoing significant nonradiative energy losses, an important property for catalytic or photovoltaic applications. The reduced rate of exciton dissociation in the CdS domain of Au/CdS nanocomposites was attributed to the nonepitaxial nature of Au/CdS interfaces associated with low defect density and a high potential barrier of the interstitial phase.

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Lambright, S.; Butaeva, E.V.; Razgoniaeva, N.; Hopkins, T.; Smith, B.; Perera, D.N.; Corbin, J.; Khon, E.; Thomas,R.; Moroz, P.; Mereshchenko, A.S.; Tarnovsky, A.N.; Zamkov, M. "Enhanced Lifetime of Excitons in Non-Epitaxial Au/CdS Core/Shell Nanocrystals." ACS Nano 2014, 8, 352

Improving the Catalytic Activity of Semiconductor Nanocrystals through Selective Domain Etching

Colloidal chemistry offers an assortment of synthetic tools for tuning the shape of semiconductor nanocrystals. While many nanocrystal architectures can be obtained directly via colloidal growth, other nanoparticle morphologies require alternative processing strategies. Here, we show that chemical etching of colloidal nanoparticles can facilitate the realization of nanocrystal shapes that are topologically inaccessible by hot-injection techniques alone. The present methodology is demonstrated by synthesizing a two-component CdSe/CdS nanoparticle dimer, constructed in a way that both CdSe and CdS semiconductor domains are exposed to the external environment. This structural morphology is highly desirable for catalytic applications as it enables both reductive and oxidative reactions to occur simultaneously on dissimilar nanoparticle surfaces. Hydrogen production tests confirmed the improved catalytic activity of CdSe/CdS dimers, which was enhanced 3–4 times upon etching treatment. We expect that the demonstrated application of etching to shaping of colloidal heteronanocrystals can become a common methodology in the synthesis of charge-separating nanocrystals, leading to advanced nanoparticles architectures for applications in areas of photocatalysis, photovoltaics, and light detection.

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Khon, E.; Lambright, K.; Khnayzer, R.S.; Moroz, P.; Perera, D.N.; Butaeva, E.; Lambright, S.; Castellano, F.N.; Zamkov, M. "Improving the Catalytic Activity of Semiconductor Nanocrystals through Selective Domain Etching." Nano Lett. 2013, 13, 2016-2023.

Fabrication of all-inorganic nanocrystal solids through matrix encapsulation of nanocrystal arrays

A general strategy for low-temperature processing of colloidal nanocrystals into all-inorganic films is reported. The present methodology goes beyond the traditional ligand-interlinking scheme and relies on encapsulation of morphologically defined nanocrystal arrays into a matrix of a wide-band gap semiconductor, which preserves optoelectronic properties of individual nanoparticles while rendering the nanocrystal film photoconductive. Fabricated solids exhibit excellent thermal stability, which is attributed to the heteroepitaxial structure of nanocrystal–matrix interfaces, and show compelling light-harvesting performance in prototype solar cells.

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Kinder, E.; Moroz, P.; Diederich, G.; Johnson, A.; Kirsanova, M.; Nemchinov, A.; O'Connor, T.; Roth, D.; Zamkov, M., "Fabrication of all-inorganic nanocrystal solids through matrix encapsulation of nanocrystal arrays." J. Am. Chem. Soc. 2011, 133, 20488–20499.

The role of hole localization in sacrificial hydrogen production by semiconductor–metal heterostructured nanocrystals

The effect of hole localization on photocatalytic activity of Pt-tipped semiconductor nanocrystals is investigated. By tuning the energy balance at the semiconductor–ligand interface, we demonstrate that hydrogen production on Pt sites is efficient only when electron-donating molecules are used for stabilizing semiconductor surfaces. These surfactants play an important role in enabling an efficient and stable reduction of water by heterostructured nanocrystals as they fill vacancies in the valence band of the semiconductor domain, preventing its degradation. In particular, we show that the energy of oxidizing holes can be efficiently transferred to a ligand moiety, leaving the semiconductor domain intact. This allows reusing the inorganic portion of the "degraded" nanocrystal-ligand system simply by recharging these nanoparticles with fresh ligands.

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Acharya, K.; Khnayzer, R. S.; O'Connor, T.; Diederich, G.; Kirsanova, M.; Klinkova, A.; Roth, D.; Kinder, E.; Imboden, M.; Zamkov, M., "The role of hole localization in sacrificial hydrogen production by semiconductor–metal heterostructured nanocrystals." Nano Lett. 2011, 11, 2919–2926.

Heteroepitaxial growth of colloidal nanocrystals onto substrate films via hot-injection routes

Hot-injection synthesis of colloidal nanocrystals (NCs) in a substrate-bound form is demonstrated. We show that polycrystalline films submerged into hot organic solvents can nucleate the heteroepitaxial growth of semiconductor NCs, for which the ensuing lattice quality and size distribution are on the par with those of isolated colloidal nanoparticles.

This strategy is demonstrated by growing lead chalcogenide NCs directly onto solvent-submerged TiO2 substrates. The resulting PbX/TiO2 (X = S, Se, Te) nanocomposites exhibit heteroepitaxial interfaces between lead chalcogenide and oxide domains and show an efficient separation of photoinduced charges, deployable for light-harvesting applications.

The extendibility of the present method to other material systems was demonstrated through the synthesis of CdS/TiO2 and Cu2S/TiO2 heterostructures, fabricated from PbS/TiO2 composites via cation exchange. The photovoltaic performance of nanocrystal/substrate composites comprising PbS NCs was evaluated by incorporating PbS/TiO2 films into prototype solar cells.

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Acharya, K. P.; Khon, E.; O'Connor, T.; Nemitz, I.; Klinkova, A.; Khnayzer, R. S.; Anzenbacher, P.; Zamkov, M., "Heteroepitaxial growth of colloidal nanocrystals onto substrate films via hot-injection routes." ACS Nano 2011, 5, 4953–4964.

Tuning the morphology of Au/CdS nanocomposites through temperature-controlled reduction of gold-oleate complexes

A general synthetic strategy for controlling the shape of gold domains grown onto CdS semiconductor nanocrystals is presented. The colloidal growth of Au nanoparticles is based on the temperature-controlled reduction of Au-oleate complexes on the surface of CdS and allows for precise tuning of nanoparticle diameters from 2.5 to 16 nm simply by adjusting the temperature of the growth solution, whereas the shape of Au/CdS nanocomposites can be controllably switched between matchsticks and barbells via the reaction rate. Depending on the exact morphology of Au and CdS domains, fabricated nanocomposites can undergo evaporation-induced self-assembly on a substrate either through end-to-end coupling of Au domains, resulting in the formation of one-dimensional chains or via side-by-side packing of CdS nanorods, leading to the onset of two-dimensional superlattices.

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Khon, E.; Hewa-Kasakarage, N. N.; Nemitz, I.; Acharya, K. P.; Zamkov, M., "Tuning the morphology of Au/CdS nanocomposites through temperature-controlled reduction of gold-oleate complexes." Chem. Mater. 2010, 22, 5929–5936.

Radiative recombination of spatially extended excitons in (ZnSe/CdS)/CdS heterostructured nanorods

We report on organometallic synthesis of luminescent (ZnSe/CdS)/CdS semiconductor heterostructured nanorods (hetero-NRs) that produce an efficient spatial separation of carriers along the main axis of the structure (type II carrier localization). Nanorods were fabricated using a seeded-type approach by nucleating the growth of 20−100 nm CdS extensions at [000 ± 1] facets of wurtzite ZnSe/CdS core/shell nanocrystals. The difference in growth rates of CdS in each of the two directions ensures that the position of ZnSe/CdS seeds in the final structure is offset from the center of hetero-NRs, resulting in a spatially asymmetric distribution of carrier wave functions along the heterostructure. Present work demonstrates a number of unique properties of (ZnSe/CdS)/CdS hetero-NRs, including enhanced magnitude of quantum confined Stark effect and subnanosecond switching of absorption energies that can find practical applications in electroabsorption switches and ultrasensitive charge detectors.

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Hewa-Kasakarage, N. N.; Kirsanova, M.; Nemchinov, A.; Schmall, N.; El-Khoury, P. Z.; Tarnovsky, A. N.; Zamkov, M., "Radiative recombination of spatially extended excitons in (ZnSe/CdS)/CdS heterostructured nanorods." J. Am. Chem. Soc. 2009, 131, 1328–1334.

Synthesis and characterization of type II ZnSe/CdS core/shell nanocrystals

High-quality ZnSe/CdS core/shell nanocrystals, exhibiting a type II carrier localization regime, were fabricated via a traditional pyrolysis of organometallic precursors. The two-step synthesis involved fabrication of 4.5−6 nm ZnSe seeds followed by a subsequent deposition of the CdS shell.

An efficient spatial separation of electrons and holes between the core and the shell was observed for heterostructures containing more than three monolayers of CdS, which was primarily evidenced by the spatially indirect emission tunable from 480 to 610 nm for a fixed core diameter. Because of a large (type II) offset of band edges at the core/shell interface, fabricated nanocrystals exhibited a relatively low spectral overlap between emission and absorption profiles, with associated Stokes shifts of up to 110 nm.

The quantum yield of as-prepared samples was 12−18% and was further improved to 20% after purification of nanocrystals through multiple hexane/methanol extractions. Novel properties of synthesized ZnSe/CdS nanocrystals as well as their applicability to practical realizations in areas of biomedical imaging, solar sells, and quantum dot-based lasers are discussed.

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Nemchinov, A.; Kirsanova, M.; Hewa-Kasakarage, N. N.; Zamkov, M., "Synthesis and characterization of type II ZnSe/CdS core/shell nanocrystals." J. Phys. Chem. C 2008, 112, 9301-9307.