Narayan Pradhan

Doping Cu in Semiconductor Nanocrystals: Some Old and Some New Physical Insights

Cu-doped inorganic semiconductors with concomitant optical properties have garnered enormous research interest in the last two decades. However, uncertainties over the origin of Cu emission, its oxidation state, resemblance with trap state emission, position of Cu d-state, emission spectral width, and moreover understanding of the doping mechanism restricted the wide development of the synthetic methodology for high-quality Cu-doped nanocrystals. It has been shown recently that the emission from Cu-doped semiconductor nanocrystals can span over a wide spectral window and could be a potential color tunable dispersed nanocrystal emitter. Herein, we report the size and composition of variable Cu-doped ZnS/Zn(1−x)Cd(x)S zinc-blende (ZB) surface alloyed nanocrystals with intense, stable, and tunable emission covering the blue to red end of the visible spectrum. Further, the Cu dopant emission is distinguished from trap state emission, and the composition variable spectral broadening has been justified on the account of a different environment around the Cu ions in the host lattice. Whereas some findings are in agreement with past reports, several new physical insights presented here would help the community for an in-depth mechanistic study on Cu doping. Moreover, these doped nanocrystal emitters can be a promising candidate for application ranging from optoelectronics to bio-labeling.

Metal Semiconductor Heterostructures for Photocatalytic Conversion of Light Energy

For fast separation of the photogenerated charge carriers, metal semiconductor heterostructures have emerged as one of the leading materials in recent years. Among these, metal Au coupled with low bandgap semiconductors remain as ideal materials where both can absorb the solar light in the visible region. It is also established that on excitation, the plasmonic state of gold interacts with excited state of semiconductor and helps for the delocalization of the photogenerated electrons. Focusing these materials where electron transfer preferably occurs from semiconductor to metal Au on excitation, in this Perspective, we report the latest developments in the synthetic chemistry in designing such nano heterostructures and discuss their photocatalytic activities in organic dye degradation/reduction and/or photocatalytic water splitting for generation of hydrogen. Among these, materials such as Au-CZTS, Au-SnS, Au-Bi2S3, Au-ZnSe, and so forth are emphasized, and their formation chemistry as well as their photocatalytic activities are discussed in this Perspective.

Photocatalytic Au–Bi2S3 Heteronanostructures†

Au-Bi2S3 heteronanostructure photocatalysts were designed in which the coupling of a metal plasmon and a semiconductor exciton aids the absorption of solar light, enhances charge separation, and results in improved catalytic activity. Furthermore, these nanostructures show a unique pattern of structural combination, with Au nanoparticles positioned at the center of Bi2S3 nanorods. The chemistry of formation of these nanostructures, their epitaxy at the junction, and their photoconductance were studied, as well as their photoresponse properties.

Ultrasmall Color-Tunable Copper-Doped Ternary Semiconductor Nanocrystal Emitters†

Synthesis of light-emitting dispersed semiconductor nanocrystals with tunable emission has been widely studied in the last two decades because of their potential applications in photovoltaics, optoelectronics, and biology. Soon after the development of high-quality CdSe nanocrystals with spectacular size-dependent tunable excitonic emission in the entire visible window, simplification of the synthetic method, stabilization of the emission, surface functionalization of the nanocrystals, design of non-cadmium nanocrystal emitters, fabrication of different kinds of composition-tunable multifunctional alloy nanocrystals, and related photophysical properties have been widely investigated for both fundamental understanding and their implementation in day-to-day developing technology. Analysis of up to date literature reports reveals that biological applications require strongly emitting, small and nontoxic nanocrystals preferably with excitation in the visible window, light-emitting diodes require nanocrystals having large Stokes shift and high quantum efficiency, and for solar cells nanocrystals having visible/near-IR (NIR) absorption and/or ternary/ quaternary nanocrystals with excess of either of the charge carriers (electron or hole) 11] are preferred. So far, no nanocrystal emitters having all such required properties have been reported, and thus further investigations are required to obtain new materials with new properties that would be suitable for versatile applications. We have now designed a new series of ultrasmall (< 2.5 nm), nearly fixed size, alloyed nanocrystals composed of Cu–Zn–In–Se ions which show composition-dependent tunable emissionover most of the visible window. In addition, these nanocrystals are cadmium-free and have aqueous dispersibility, photostability, large Stokes shifts, and high emission intensity (quantum yield (QY) = 25–30%), which makes them a versatile light-emitting nanoscale materials providing one-step solutions for various applications. The fundamental designing principle of these nanocrystals involves a mechanism whereby composition-variable alloy formation tunes the optical bands from lower to higher energy and vice versa. Here we report details of the synthesis, chemistry of formation, and composition-variable optical tuning of these fixed-size alloy nanocrystals. In addition, aqueous dispersibility and photovoltaic properties of these nanocrystals were investigated. The alloy nanocrystals were synthesized by simultaneous precipitation and surface cation-exchange protocols. Injection of a selenium precursor into a mixture of Zn, In, and Cu salts at 220 8C (see Experimental Section) results in copperdoped zinc indium selenide alloy nanocrystals whose absorption and emission wavelengths are determined by the In:Zn ratio of the reaction mixture. Further addition of Zn with continuous annealing slowly shifts both absorption and emission bands to the blue in a surface ion-exchange process. Successive photoluminescence (PL) spectra, absorption (UV/ Vis) spectra, and a schematic model of surface cation exchange for a typical alloying process are shown in Figure 1a–c. With an initial Zn:In ratio of 1:2, the emission appears at about 660 nm soon after injection of the Se precursor and is tuned up to 575 nm (Figure 1a) on introduction of additional Zn precursor, while for an initial Zn:In ratio of 1:1, the emission appears at about 620 nm and is tuned further to the blue, to 540 nm (Figure 1 b), that is, a total

Prevention of photooxidation in blue-green emitting Cu doped ZnSe nanocrystals

This communication highlights unstable blue-green emitting Cu doped ZnSe nanocrystals stabilized by diluting the surface Se with a calculated amount of S.

Mn-Doped Multinary CIZS and AIZS Nanocrystals

Multinary nanocrystals (CuInS2, CIS, and AgInS2, AIS) are widely known for their strong defect state emission. On alloying with Zn (CIZS and AIZS), stable and intense emission tunable in visible and NIR windows has already been achieved. In these nanocrystals, the photogenerated hole efficiently moves to the defect-induced state and recombines with the electron in the conduction band. As a result, the defect state emission is predominantly observed without any band edge excitonic emission. Herein, we report the doping of the transition-metal ion Mn in these nanocrystals, which in certain compositions of the host nanocrystals quenches this strong defect state emission and predominantly shows the spin-flip Mn emission. Though several Mn-doped semiconductor nanocrystals are reported in the literature, these nanocrystals are of its first kind that can be excited in the visible window, do not contain the toxic element Cd, and provide efficient emission. Hence, when Mn emission is required, these multinary nanocrystals can be the ideal versatile materials for widespread technological applications.

Multifunctional Doped Semiconductor Nanocrystals

Multifunctional nanomaterials with combined magnetic and optical properties remain one of the most demanded materials in upcoming research. To obtain these materials, we report here several doped semiconductor nanocrystals that simultaneously show tunable emission in a visible and NIR spectral window, above-room-temperature ferromagnetism, and improved conductivity. These nanocrystals are designed by inserting Ni(II) as a dopant in various semiconducting hosts with binary, alloyed, and ternary composition, and the induced multifunctional properties are observed to be stable and reproducible. These semiconducting materials combined with fluorescence and magnetic properties would be useful for a wide range of applications spanning from life science to modern developing device technology.

Tuning the Emission Colors of Semiconductor Nanocrystals Beyond their Bandgap Tunability: All in the Dope

Adopting the concept of one dopant for one color, all the prominent emitting colors in the visible windows are obtained by doping selective dopants (Ag, Cu, Ni, and Cu) in an appropriate host (alloy of Cdx Zn1-x S) with fixed size/composition and bandgap. Analyzing the origin of these emissions the relative position of respective dopant states are correlated.

Formation of Heteroepitaxy in Different Shapes of Au–CdSe Metal–Semiconductor Hybrid Nanostructures

Formation of heteroepitaxy and designing different-shaped heterostructured nanomaterials of metal and semiconductor in solution remains a frontier area of research. However, it is evident that the synthesis of such materials is not straightforward and needs a selective approach to retain both metal and semiconductor identities in the reaction system during heterostructure formation. Herein, the epitaxial growth of semiconductor CdSe on selected facets of metal Au seeds is reported and different shapes (flower, tetrapod, and core/shell) hetero-nanostructures are designed. These results are achieved by controlling the reaction parameters, and by changing the sequence and timing for introduction of different reactant precursors. Direct evidence of the formation of heteroepitaxy between {111} facets of Au and (0001) of wurtzite CdSe is observed during the formation of these three heterostructures. The mechanism of the evolution of these hetero-nanostructures and formation of their heteroepitaxy with the planes having minimum lattice mismatch are also discussed. This shape-control growth mechanism in hetero-nanostructures should be helpful to provide more information for establishing the fundamental study of heteroepitaxial growth for designing new nanomaterials. Such metal-semiconductor nanostructures may have great potential for nonlinear optical properties, in photovoltaic devices, and as chemical sensors.

Luminescence, Plasmonic, and Magnetic Properties of Doped Semiconductor Nanocrystals

Introducing a few atoms of impurities or dopants in semiconductor nanocrystals can drastically alter the existing properties or even introduce new properties. For example, mid-gap states created by doping tremendously affect photocatalytic activities and surface controlled redox reactions, generate new emission centers, show thermometric optical switching, make FRET donors by enhancing the excited state lifetime, and also create localized surface plasmon resonance induced low energy absorption. In addition, researchers have more recently started focusing their attention on doped nanocrystals as an important and alternative material for solar energy conversion to meet the current demand for renewable energy. Moreover, the electrical and magnetic properties of the host are also strongly altered on doping. These beneficial dopant-induced changes suggest that doped nanocrystals with proper selections of dopant-host pairs may be helpful for generating designer materials for a wide range of current technological needs. How properties relate to the doping of a variety of semiconductor nanocrystals are summarized in this Review.