Hybrid epitaxial-colloidal semiconductor nanostructures

Doping MAPbBr3 hybrid perovskites with CdSe/CdZnS quantum dots: from emissive thin films to hybrid single-photon sources

We report the first doping of crystalline methyl-ammonium lead bromide perovskite (MAPbBr₃) films with CdSe/CdZnS core/shell quantum dots (QDs), using a soft-chemistry approach that preserves their high quantum yield and other remarkable luminescence properties. Our approach produces MAPbBr₃ films of around 100 nm thickness, doped at volume ratios between 0.01 and 1% with colloidal CdSe/CdZnS QDs whose organic ligands were exchanged with halide ions to allow for close contact between the QDs and the perovskite matrix. Ensemble photoluminescence (PL) measurements demonstrate the retained emission of the QDs after incorporation into the MAPbBr₃ matrix. Photoluminescence excitation (PLE) spectra exhibit signatures of wavelength-dependent coupling between the CdSe/CdZnS QDs and the MAPbBr₃ matrix, i.e., a transfer of charges from matrix to QD, which increases the QD luminescence by up to 150%, or from QD to matrix. Spatially-resolved PL experiments reveal a strong correlation between the positions of QDs and an enhancement of the PL signal of the matrix. Lifetime imaging of the doped films furthermore shows that the emission lifetime of MAPbBr₃ is slower in the vicinity of QDs, which, in combination with the increased PL signal of the matrix, suggests that QDs can act as local nucleation seeds that improve the crystallinity of MAPbBr₃, thus boosting its emission quantum yield. Luminescence antibunching measurements provide clear evidence of single-photon emission from individual QDs in perovskite. Finally, the analysis of blinking statistics indicates an improvement of the photostability of individual QDs in perovskite as compared to bare CdSe/CdZnS QDs. At high CdSe/CdZnS QD doping levels, this work thus opens a route to hybrid solar concentrators for visible-light harvesting and hybrid-based LEDs, while a low degree of doping could yield hybrid single-photon sources than can be embedded in field-effect devices for single-charge control, which would allow the construction of nanophotonic devices via low-cost solution-processing techniques as an alternative to solid-state quantum dots.

In Situ Photoluminescence of Colloidal Quantum Dots During Gas Exposure-The Role of Water and Reactive Atomic Layer Deposition Precursors

Colloidal quantum dots (QDs) are a promising material for optoelectronic applications. Typically, device integration requires QDs to be embedded in a host material. Atomic layer deposition (ALD) is often considered as a deposition technique for such purposes. However, it is known that ALD and vacuum processes often influence the optical properties of QDs in a negative way. Here, we describe an in situ photoluminescence (PL) measurement setup and use it to monitor the PL of QDs under vacuum and during ALD. For CdSe-based core/shell QDs, a reduction in the QD PL was observed upon exposure to vacuum. Water was identified as crucial for maintaining a high PL as evidenced by re-exposure to different gases. Furthermore, we addressed the influence of vacuum, different plasmas (O₂, H₂O, H₂, H₂S/Ar, and Ar), precursors (trimethylaluminum, diethylzinc, tetrakis(dimethylamido)titanium, and tetrakis(ethylmethylamido)hafnium), reactants (H₂O, H₂S, and O₃), and ALD processes (Al₂O₃, TiO₂, HfO₂, and ZnS) on QDs. We observed a PL reduction by up to 90% upon plasma treatments. Furthermore, we found that trimethylaluminum and diethylzinc reduced the PL efficiency by more than 70% while exposure to tetrakis(dimethylamido)titanium and tetrakis(ethylmethylamido)hafnium lowered the PL by only 10-20%. Surprisingly, tetrakis(dimethylamido)titanium and H₂O, which by themselves had only a minor influence on the QD PL, still caused an 80% drop of the PL efficiency when combined as an ALD process. On the other hand, ALD growth of HfO₂ by combining tetrakis(ethylmethylamido)hafnium and O₃ preserved 80% of the initial PL quantum yield, making it a promising process for QD embedding. These results put forward in situ PL measurements as a versatile technique to identify suitable precursors, reactants and ALD processes for QD embedding and investigate the interaction between QDs and reactive gaseous species in general.

Nondestructive Encapsulation of CdSe/CdS Quantum Dots in an Inorganic Matrix by Pulsed Laser Deposition

We report the successful encapsulation of colloidal quantum dots in an inorganic matrix by pulsed laser deposition. Our technique is nondestructive and thus permits the incorporation of CdSe/CdS core/shell colloidal quantum dots in an amorphous yttrium oxide matrix (Y2O3) under full preservation of the advantageous optical properties of the nanocrystals. We find that controlling the kinetic energy of the matrix precursors by means of the oxygen pressure in the deposition chamber facilitates the survival of the encapsulated species, whose well-conserved optical properties such as emission intensity, luminescence spectrum, fluorescence lifetime, and efficiency as single-photon emitters we document in detail. Our method can be extended to different types of nanoemitters (e.g., nanorods, dots-in-rods, nanoplatelets) as well as to other matrices (oxides, semiconductors, metals), opening up new vistas for the realization of fully inorganic multilayered active devices based on colloidal nano-objects.

General synthetic approach to heterostructured nanocrystals based on noble metals and I-VI, II-VI, and I-III-VI metal chalcogenides

Solid metal precursors (alloys or monometals) can serve both as a starting template and as a source material for chemical transformation to metal chalcogenides. Herein, we develop a simple solution-based strategy to obtain highly monodisperse noble-metal-based heterostructured nanocrystals from such precursor seeds. By utilizing chemical and structural inhomogeneity of these metal seeds, in this work, we have synthesized a total of five I-VI (Ag2S, Ag2Se, Ag3AuS2, Ag3AuSe2, and Cu9S5), three II-VI (CdS, CdSe, and CuSe), and four I-III-VI (AgInS2, AgInSe2, CuInS2, and CuInSe2) chalcogenides, together with their fifteen associated heterodimers (Au-Ag2S, Au-Ag2Se, Au-Ag3AuS2, Au-Ag3AuSe2, Au-AgInS2, Au-AgInSe2, Au-CdS, Au-CdSe, Ag-Ag2S, Ag-AgInS2, Au-Cu9S5, Au-CuInS2, Au-CuSe, Au-CuInSe2, and Pt-AgInS2) to affirm the process generality. Briefly, by adding elemental sulfur or selenium to AuAg alloy seeds and tuning the reaction conditions, we can readily obtain phase-pure Au-Ag2S, Au-Ag2Se, Au-Ag3AuS2, and Au-Ag3AuSe2 heterostructures. Similarly, we can also fabricate Au-AgInS2 and Au-AgInSe2 heterostructures from the AuAg seeds by adding sulfur/selenium and indium precursors. Furthermore, by partial or full conversion of Ag seeds, we can prepare both single-phase Ag chalcogenide nanocrystals and Ag-based heterostructures. To demonstrate wide applicability of this strategy, we have also synthesized Au-based binary and ternary Cu chalcogenide (Au-Cu9S5, Au-CuSe, Au-CuInS2, and Au-CuInSe2) heterostructures from alloy seeds of AuCu and Pt chalcogenides (e.g., Pt-AgInS2) from alloy seeds of PtAg. The structure and composition of the above products have been confirmed with X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and energy-dispersive X-ray spectroscopy methods. A kinetic investigation of the formation mechanism of these heterostructures is brought forward using Au-AgInS2 and Ag-CuInS2 as model examples.

Constructing functional mesostructured materials from colloidal nanocrystal building blocks

Through synthesizing colloidal nanocrystals (NCs) in the organic phase, chemists gain fine control over their composition, size, and shape. Strategies for arranging them into ordered superlattices have followed closely behind synthetic advances. Nonetheless, the same hydrophobic ligands that help their assembly also severely limit interactions between adjacent nanocrystals. As a result, examples of nanocrystal-based materials whose functionality derives from their mesoscale structure have lagged well behind advances in synthesis and assembly. In this Account, we describe how recent insights into NC surface chemistry have fueled dramatic progress in functional mesostructures. In these constructs, intimate contact between NCs as well as with heterogeneous components is key in determining macroscopic behavior. The simplest mesoscale assemblies we consider are networks of NCs constructed by in situ replacement of their bulky, insulating surface ligands with small molecules. Transistors are a test bed for understanding conductivity, setting the stage for new functionality. For instance, we demonstrated that by electrochemically charging and discharging networks of plasmonic metal oxide NCs, the transmittance of near infrared light can be strongly and reversibly modulated. When we assemble NCs with heterogeneous components, there is an even greater potential for generating complex functionality. Nanocomposites can exhibit favorable characteristics of their component materials, yet the interaction between components can also have a strong influence. Realizing such opportunities requires an intimate linking of embedded NCs to the surrounding matrix phase. We accomplish this link by coordinating inorganic anionic clusters directly to NC surfaces. By exploiting this connection, we found enhanced ionic conductivity in Ag2S-in-GeS2 nanocrystal-in-glass electrodes. In another example, we also found enhanced optical contrast when linking electrochromic niobium oxide to embedded tin-doped indium oxide (ITO) NCs. These dramatic effects emerge from reconstruction of the inorganic glass immediately adjacent to the NC interface. When co-assembling NCs with block copolymers, direct coordination of the polymer to NC surfaces again opens new opportunities for functional mesoscale constructs. We strip NCs of their native ligands and design block copolymers containing a NC tethering domain that bonds strongly, yet dynamically, to the resulting open coordination sites. This strategy enables their co-assembly at high volume fractions of NCs and leads to well-ordered mesoporous NC networks. We find these architectures to be exceptionally stable under chemical transformations driven by cation insertion, removal, and exchange. These developments offer a modular toolbox for arranging NCs deliberately with respect to heterogeneous elements and open space. We have control over metrics that define such architectures from the atomic scale (bonding and crystal structure) through the mesoscale (crystallite shapes and sizes and pore dimensions). By tuning these parameters and better understanding the interactions between components, we look forward to boundless opportunities to employ mesoscale structure, in tandem with composition, to develop functional materials.

Improving carrier injection in colloidal CdSe nanocrystals by embedding them in a pseudomorphic ZnSe/ZnMgSe quantum well structure

The incorporation of colloidal nanocrystals in a high crystalline quality semiconductor matrix, the efficient carrier injection into the embedded nanocrystals and the fast optical response are key features for the fabrication of novel optoelectronic nanodevices based on colloidal nanostructures as active optical material. Using a novel growth approach, colloidal bare CdSe and core-shell CdSe/ZnS nanocrystals were monolithically incorporated in pseudomorphic ZnSe/ZnMgSe quantum wells in order to control and enhance the carrier transfer into the nanocrystals. The photoluminescence for bare CdSe nanocrystals incorporated in ZnSe/ZnMgSe quantum well structures is substantially enhanced in comparison to nanocrystals sandwiched in ZnSe epilayers, which we attribute to increased carrier injection into the embedded nanocrystals via the quantum well, resembling the function of a wetting layer in Stranski-Krastanov-grown quantum dots. Core-shell CdSe/ZnS nanocrystals embedded in quantum well structures do not show considerable PL modifications because the ZnS shell prevents the efficient carrier migration between the nanocrystal and the matrix. Systematic investigations of structural and optical properties by high-resolution x-ray diffraction, temperature-dependent photoluminescence and time-resolved emission are presented.

A facile method for gold decoration of Te@CdTe nanorods in aqueous solution

Colloidal synthesis of metal-semiconductor hybrid nanostructures is mainly achieved in organic solution. In some applications of hybrid nanoparticles relevant in aqueous media, phase transfer of hydrophobic metal-semiconductor hybrid nanostructures is essential. In this work, we present a simple method for direct synthesis of water-soluble gold (Au) decorated Te@CdTe hybrid nanorods (NRs) at room temperature by using aqueous Te@CdTe NRs as templates, which were preformed by using CdTe nanocrystals (NCs) as precursor in the presence of hydrazine hydrate (N(2)H(4)). Our results showed that NRs were decorated with Au islands both on tips and along the surface of the NRs. The size and density of Au islands can be controlled by varying the amount of Au precursor (mixture of HAuCl(4) and thioglycolic acid (TGA)) and TGA/HAuCl(4) ratio. A possible growth mechanism for the Au decoration of Te@CdTe NRs is concluded as three steps: (1) the formation of AuTe(1.7) via the substitution reaction of Cd(2+) by Au(3+), (2) adsorption of Au-TGA complex onto the preformed AuTe(1.7) anchors and following reduction by CdTe and N(2)H(4), leading to the formation of small Au NCs, (3) Au NCs grow to bigger ones, followed by reduction of more Au precursor by N(2)H(4).

Luminescence from single CdSe nanocrystals embedded in ZnO thin films using atomic layer deposition

We report the embedding of CdSe/ZnS core-shell nanocrystals into thin films of ZnO grown by atomic layer deposition. Fluorescence from ensembles and that from individual nanocrystals show that the nanocrystal luminescence intensity and stability are preserved through the embedding process. These results are encouraging for the fabrication of optoelectronic nanodevices based on colloidal nanoparticles.

The synthesis and fabrication of one-dimensional nanoscale heterojunctions

There are a variety of methods for synthesizing or fabricating one-dimensional (1D) nanostructures containing heterojunctions between different materials. Here we review recent developments in the synthesis and fabrication of heterojunctions formed between different materials within the same 1D nanostructure or between different 1D nanostructures composed of different materials. Structures containing 1D nanoscale heterojunctions exhibit interesting chemistry as well as size, shape, and material-dependent properties that are unique when compared to single-component materials. This leads to new or enhanced properties or multifunctionality useful for a variety of applications in electronics, photonics, catalysis, and sensing, for example. This review separates the methods into vapor-phase synthesis, solution-phase synthesis, template-based synthesis, and other approaches, such as lithography, electrospinning, and assembly. These methods are used to form a variety of heterojunctions, including segmented, core/shell, branched, or crossed, from different combinations of semiconductor, metal, carbon, and polymeric materials.

Semiconductor-metal nanoparticle molecules: hybrid excitons and the nonlinear fano effect

Modern nanotechnology opens the possibility of combining nanocrystals of various materials with very different characteristics in one superstructure. Here we study theoretically the optical properties of hybrid molecules composed of semiconductor and metal nanoparticles. Excitons and plasmons in such a hybrid molecule become strongly coupled and demonstrate novel properties. At low incident light intensity, the exciton peak in the absorption spectrum is broadened and shifted due to incoherent and coherent interactions between metal and semiconductor nanoparticles. At high light intensity, the absorption spectrum demonstrates a surprising, strongly asymmetric shape. This shape originates from the coherent internanoparticle Coulomb interaction and can be viewed as a nonlinear Fano effect which is quite different from the usual linear Fano resonance.

Uncoated, Broad Fluorescent, and Size-Homogeneous CdSe Quantum Dots for Bioanalyses

In the present study, we describe the synthesis of highly luminescent uncoated water-soluble CdSe quantum dots (QDs) possessing the following characteristics: approximately 2 nm in diameter, with very good size distribution (in 95% homodispersed) accompanied by a broad-band photoluminescent spectrum. The synthetic procedure is simple, is conducted at room temperature, in the absence of the most popular coordinating ligands (as TOPO or HDA), and is highly reproducible. The obtained CdSe core QDs possessed a comparatively long fluorescence half-life (approximately 30-90 ns, depending on the emission wavelength) detected by time-resolved spectroscopy. These QDs were further conjugated with antibodies and applied in several biochemical analyses.