46 research outputs found
Colloidal Atomic Layer Deposition (c-ALD) using Self-Limiting Reactions at Nanocrystal Surface Coupled to Phase Transfer between Polar and Nonpolar Media
Atomic layer deposition (ALD) is widely used for gas-phase
deposition
of high-quality dielectric, semiconducting, or metallic films on various
substrates. In this contribution we propose the concept of colloidal
ALD (c-ALD) for synthesis of colloidal nanostructures. During the
c-ALD process, either nanoparticles or molecular precursors are sequentially
transferred between polar and nonpolar phases to prevent accumulation
of unreacted precursors and byproducts in the reaction mixture. We
show that binding of inorganic ligands (e.g., S<sup>2ā</sup>) to the nanocrystal surface can be used as a half-reaction in c-ALD
process. The utility of this approach has been demonstrated by growing
CdS layers on colloidal CdSe nanocrystals, nanoplatelets, and CdS
nanorods. The CdS/CdSe/CdS nanoplatelets represent a new example of
colloidal nanoheterostructures with mixed confinement regimes for
electrons and holes. In these materials holes are confined to a thin
(ā¼1.8 nm) two-dimensional CdSe quantum well, while the electron
confinement can be gradually relaxed in all three dimensions by growing
epitaxial CdS layers on both sides of the quantum well. The relaxation
of the electron confinement energy caused a shift of the emission
band from 510 to 665 nm with unusually small inhomogeneous broadening
of the emission spectra
Colloidal CdSe Quantum Rings
Semiconductor quantum rings are of
great fundamental interest because
their non-trivial topology creates novel physical properties. At the
same time, toroidal topology is difficult to achieve for colloidal
nanocrystals and epitaxially grown semiconductor nanostructures. In
this work, we introduce the synthesis of luminescent colloidal CdSe
nanorings and nanostructures with double and triple toroidal topology.
The nanorings form during controlled etching and rearrangement of
two-dimensional nanoplatelets. We discuss a possible mechanism of
the transformation of nanoplatelets into nanorings and potential utility
of colloidal nanorings for magneto-optical (e.g., AharonovāBohm
effect) and other applications
Many-Body Effects in Nanocrystal Superlattices: Departure from Sphere Packing Explains Stability of Binary Phases
This work analyzes
the role of hydrocarbon ligands in the self-assembly
of nanocrystal (NC) superlattices. Typical NCs, composed of an inorganic
core of radius <i>R</i> and a layer of capping ligands with
length <i>L</i>, can be described as soft spheres with softness
parameter <i>L</i>/<i>R</i>. Using particle tracking
measurements of transmission electron microscopy images, we find that
close-packed NCs, like their hard-sphere counterparts, fill space
at approximately 74% density independent of softness. We uncover deformability
of the ligand capping layer that leads to variable effective NC size
in response to the coordination environment. This effect plays an
important role in the packing of particles in binary nanocrystal superlattices
(BNSLs). Measurements on BNSLs composed of NCs of varying softness
in several coordination geometries indicate that NCs deform to produce
dense BNSLs that would otherwise be low-density arrangements if the
particles remained spherical. Consequently, rationalizing the mixing
of two NC species during BNSL self-assembly need not employ complex
energetic interactions. We summarize our analysis in a set of packing
rules. These findings contribute to a general understanding of entropic
effects during crystallization of deformable objects (e.g., nanoparticles,
micelles, globular proteins) that can adapt their shape to the local
coordination environment
Self-Assembly of Tetrahedral CdSe Nanocrystals: Effective āPatchinessā via Anisotropic Steric Interaction
Controlling the spontaneous
organization of nanoscale objects remains
a fundamental challenge of materials design. Here we present the first
characterization of self-assembled superlattices (SLs) comprised of
tetrahedral nanocrystal (NCs). We observe self-assembly of CdSe nanotetrahedra
into an open structure (estimated space-filling fraction Ļ ā
0.59) which has not been anticipated by many recent theoretical studies
and simulations of tetrahedron packings. This finding highlights a
gap in the understanding of the hierarchy of energy scales acting
on colloidal NCs during self-assembly. We propose a strong dependence
of ligandāligand interaction potential on NC surface curvature.
This effect favors spatial proximity of vertices in the dense colloidal
crystal and may be considered an emergent āpatchinessā
acting through chemically identical ligand molecules
Role of Precursor Reactivity in Crystallization of Solution-Processed Semiconductors: The Case of Cu<sub>2</sub>ZnSnS<sub>4</sub>
We
study the formation of Cu<sub>2</sub>ZnSnS<sub>4</sub> (CZTS)
films from various liquid-phase precursors. Our experimental data
point to the significant role that reactivities of precursor components
play in the quality of the final material. Although reactive molecular
precursors favor formation of CZTS under milder conditions, the formation
of large crystalline domains requires using less reactive nanostructured
precursors. We explain this effect using kinetics of nucleation and
growth. We have also demonstrated a strategy to effectively enhance
grain growth of CZTS using solid-state phase transition as the driving
force for nanocrystal sintering. We hope this contribution will provide
a useful guide toward the rational design of liquid-phase precursors
for inorganic semiconductors for electronic and optoelectronic applications
IIIāV Nanocrystals Capped with Molecular Metal Chalcogenide Ligands: High Electron Mobility and Ambipolar Photoresponse
In this work, we synthesized InP and InAs nanocrystals
(NCs) capped
with different inorganic ligands, including various molecular metal
chalcogenide complexes (MCCs) and chalcogenide ions. We found that
MCCs and chalcogenide ions can quantitatively displace organic ligands
from the surface of IIIāV NCs and serve as the inorganic capping
groups for IIIāV NC surfaces. These inorganic ligands stabilize
colloidal solutions of InP and InAs NCs in polar solvents and greatly
facilitate charge transport between individual NCs. Charge transport
studies revealed high electron mobility in the films of MCC-capped
InP and InAs NCs. For example, we found that bridging InAs NCs with
Cu<sub>7</sub>S<sub>4</sub><sup>ā</sup> MCC ligands can lead
to very high electron mobility exceeding 15 cm<sup>2</sup>/(V s).
In addition, we observed unprecedented ambipolar (positive/negative)
photoresponse of MCC-capped InAs NC solids that changed sign depending
on the ligand chemistry, illumination wavelength, and doping of the
NC solid. For example, the sign of photoconductance of InAs NCs capped
with Cu<sub>7</sub>S<sub>4</sub><sup>ā</sup> or Sn<sub>2</sub>S<sub>6</sub><sup>4ā</sup> ions converted from positive at
0.80 and 0.95 eV to negative at 1.27 and 1.91 eV. We propose an explanation
of this unusually complex photoconductivity of InAs NC solids
Structural Defects in Periodic and Quasicrystalline Binary Nanocrystal Superlattices
Binary nanocrystal superlattices (BNSLs) emerge as an important class of man-made materials where components and functionalities can be added, tuned, or combined in a predictable manner. These amazingly complex structures spontaneously self-assemble from colloidal solutions containing binary mixtures of functional (semiconducting, magnetic, plasmonic, etc.) nanocrystals. Further developments of the BNSL-based materials require a deep understanding and control over BNSL formation and structural perfection. Like any solid, BNSL can contain different kinds of structural defects. It is well-known that defects can have a tremendous effect on the materialās behavior. Defect engineering is used to modify and improve many of the mechanical, electrical, magnetic, and optical properties of conventional solids. In this work, we provide the first systematic analysis of structural defects in various BNSL structures. We used BNSLs as a platform for studying structural defects in both periodic (crystalline) and aperiodic (quasicrystalline) lattices, as well as for direct imaging of the interfaces between crystalline and quasicrystalline domains. Such direct observation of local imperfections in complex multicomponent lattices provides a unique insight into the fundamental aspects of crystal formation
Violet-to-Blue Gain and Lasing from Colloidal CdS Nanoplatelets: Low-Threshold Stimulated Emission Despite Low Photoluminescence Quantum Yield
Amplified spontaneous emission (ASE)
and lasing from solution-processed
materials are demonstrated in the challenging violet-to-blue (430ā490
nm) spectral region for colloidal nanoplatelets of CdS and newly synthesized
core/shell CdS/ZnS nanoplatelets. Despite modest band-edge photoluminescence
quantum yields of 2% or less for single excitons, which we show results
from hole trapping, the samples exhibit low ASE thresholds. Furthermore,
four-monolayer CdS samples show ASE at shorter wavelengths than any
reported film of colloidal quantum-confined material. This work underlines
that low quantum yields for single excitons do not necessarily lead
to a poor gain medium. The low ASE thresholds originate from negligible
dispersion in thickness, large absorption cross sections of 2.8 Ć
10<sup>ā14</sup> cm<sup>ā2</sup>, and rather slow (150
to 300 ps) biexciton recombination. We show that under higher-fluence
excitation, ASE can kinetically outcompete hole trapping. Using nanoplatelets
as the gain medium, lasing is observed in a linear optical cavity.
This work confirms the fundamental advantages of colloidal quantum
well structures as gain media, even in the absence of high photoluminescence
efficiency
Orbitals, Occupation Numbers, and Band Structure of Short One-Dimensional Cadmium Telluride Polymers
Recent work found
that soldering CdTe quantum dots together with
a molecular CdTe polymer yielded field-effect transistors with much
greater electron mobility than quantum dots alone. We present a computational
study of the CdTe polymer using the active-space variational two-electron
reduced density matrix (2-RDM) method. While analogous complete active-space
self-consistent field (CASSCF) methods scale exponentially with the
number of active orbitals, the active-space variational 2-RDM method
exhibits polynomial scaling. A CASSCF calculation using the (48o,64e)
active space studied in this paper requires 10<sup>24</sup> determinants
and is therefore intractable, while the variational 2-RDM method in
the same active space requires only 2.1 Ć 10<sup>7</sup> variables.
Natural orbitals, natural-orbital occupations, charge gaps, and Mulliken
charges are reported as a function of polymer length. The polymer,
we find, is strongly correlated, despite possessing a simple sp<sup>3</sup>-hybridized bonding scheme. Calculations reveal the formation
of a nearly saturated valence band as the polymer grows and a charge
gap that decreases sharply with polymer length
Colloidal Nanocrystals with Inorganic Halide, Pseudohalide, and Halometallate Ligands
We investigate simple halides and pseudohalides as an important class of inorganic ligands for nanocrystals (NCs) in solution phase ligand exchange. These short, robust, and easy to model ligands bind to the NC surface and provide electrostatic stabilization of NC dispersions in <i>N</i>-methylformamide. The replacement of organic ligands on NCs with compact halide and pseudohalide ligands greatly facilitates electronic communication between NCs. For example, a high electron mobility of Ī¼ ā 12 cm<sup>2</sup> V<sup>ā1</sup> s<sup>ā1</sup> has been observed in thin films made of I<sup>ā</sup>-capped CdSe NCs. We also studied charge transport properties of thin films based on the pseudohalide N<sub>3</sub><sup>ā</sup>-capped InAs NCs, suggesting the possibility of obtaining āall IIIāVā NC solids. In addition, we extend the surface chemistry of halometallates (<i>e.g.,</i> CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>), which can stabilize colloidal solutions of lead chalcogenide NCs. These halide, pseudohalide, and halometallate ligands enrich the current family of inorganic ligands and can open up more opportunities for applications of NCs in the fields of electronics, optoelectronics, and thermoelectrics