Colloidal nanocrystal quantum dot assemblies as artificial solids
(Color online) Basic schematic of a NQD thin film device. The NQD building blocks are simplified as monodisperse spheres in a close-packed ordered assembly. The details shape of the crystal core and surface chemistry of the crystal facets are omitted for simplicity.
(Color online) (a) Basic model of colloidal NQD with stabilizing ligands. (b) Detailed molecular dynamics model of NQD with full ligand shell, and (c) high-resolution TEM image showing crystal shape and faceting. [Image credit: W. Baumgardner and L. Fitting-Kourkoutis (unpublished)].
(Color online) Schematic illustration of (a) classical solid comprised of atoms (i.e., carbon atoms in a diamond lattice) and (b) analogous artificial solid comprised of NQD as artificial atoms.
(Color online) Evolution of the electronic structure from (a) discrete energy levels NQD as artificial atoms to (b) hybridized energy levels in artificial molecules and (c) continuous energy bands in NQD artificial solids. [Energy levels in are based on effective mass calculations by J. Yang and F. Wise (unpublished)].
Quantized electronic states in PbS NQD. (a) Absorption spectrum and calculated transition strengths of 8.5 nm PbS NQDs and (b) density of states in an ideal and real NQD compared to the density of states in a bulk solid. [From F. Wise, Acc. Chem. Res. 33, 773 (2000).]
(Color online) Electronic coupling between proximate NQDs. (a) Quantum mechanical coupling arises from overlap of the envelope wave functions. The coupling energy depends exponentially on the interdot separation and the height of the energy barrier. (b) Electrostatic charging energy (Ec ) arises from the Coulombic energy penalty to add or remove a charge from the NQD.
(Color online) Theoretical analysis of miniband formation in NQD solids. (a) Schematic of the three-dimensionally ordered quantum dot superlattice showing notations for the quantum dot sizes and interdot spacing [from Nika et al., Phys Rev B 76, 125417 (2007)]. (b) Miniband width as a function of interdot spacing, H [from O. Lazarenkova and A. Balandin, J. Appl. Phys. 89, 5509 (2001)].
(Color online) Electronic structure of NQD solids (a) schematic phase diagram of electronic states as a function of coupling strength and disorder (b) Illustration of localized and delocalized states in a hexagonal 2D array of NQD [adopted from Remacle and Levine, ChemPhysChem 2, 20 (2001)].
(Color online) Elementary operating steps in a NQD thin film device. Photoexcitation and charge separation in a NQD solar cell (a) schematic and (b) TEM image of PbSe NQDs. (c) Illustration of subprocesses of photovoltaic cell including (i) photoexcitation, (ii) thermalization of hot carriers to the lowest unoccupied state, (iii) desirable charge transfer across the NQD boundary, (iv) undesirable trapping in surface states, and (v) recombination. Other ultrafast processes, such as hot electron transfer or multiexciton generation are omitted for clarity; these processes are discussed in the text.
(Color online) Colloidal NQD synthesis (a) basic wet synthesis experimental setup, (b) conversion of molecular precursors to nanocrystal nuclei and fully developed nanocrystals, and (c) typical concentration and temperature profiles illustrating the temporal separation of nucleation and growth.
Size-tunable energy gap. Room temperature optical absorption spectra of (a) CdSe NQD ranging in size from 1.2 to 11.5 nm [from Murray et al., J. Am. Chem. Soc. 115, 8706 (1993)] and (b) PbSe NQD ranging in size from 3 to 9 nm [from Murray et al., IBM J. Res. Dev. 45, 47 (2001)].
(Color online) Structural diversity of colloidal semiconductor NQD (a) PbSe NQD shaped as spheres, octahedral and stars [from Houtepen et al., J. Am. Chem. Soc. 128, 6792 (2006)]. (b) 1D PbSe nanocrstructures formed by dipole-mediated alignment [from Cho et al., J. Am. Chem. Soc. 127, 7140 (2005)]. (c) Multicomponent core/shell structure [Pandey and Guyot-Sionnest, Science 322, 929 (2008)] and (d) 1D CdSe/CdS heterostructured [from Becker et al., Nat. Mater 5, 777 (2006)].
(Color online) Size-tuned energy level offsets in NQD solar cells. (a) Colloidal NQD synthesis (a) PbSe NQD energy levels measured by CV (black dots). Measured energy levels show good agreement with theoretical calculation. The red line (top) shows the electron affinity of ZnO and the blue line (bottom) shows ionization potential of the hole conductor PEDOT:PSS. (b) Open circuit voltages from devices (V oc) with various PbSe NC sizes. Two distinctive domains are observed. Devices made from large (d > ∼3.8 nm) NCs show no appreciable photovoltaic effect whereas devices made from small (d < ∼3.8 nm) NCs show increasing V oc with decreasing NC size. The critical diameter (∼4 nm), which defines the transition between the “linear” and “off” domains, agrees reasonably with the CV result. The linear fits (dotted lines) to the respective domains provide information on the minimum interfacial energy level offset required for charge separation PbSe NQD-PV. [From Choi et al., Nano Lett. 9, 3749 (2009)].
(Color online) Illustration of various molecular interdot coupling configurations. (a) basic ligand (e.g., ethanethiol), (b) symmetric bidentate linker (e.g., 1,2-ethanedithiol), (c) asymmetric linker (e.g., mercaptopropionic acid), and (d) aromatic linker (e.g., 1,4-benzenedithiol).
(Color online) Tunable artificial bond length realized through variable length bifunctional linkers. (a) Charge transfer rates as a function of interdot spacing measured with GISAXS (top panel). Single exponential decay fit (black solid line) indicates that the charge transfer occurs via tunneling of charge through a potential barrier. Resonant energy transfer is shown in the oleic acid passivated NQD (black rectangle) and the calculated energy transfer rates (dotted line) for corresponding interdot spacing using a Förster radius of 5 nm shows an order of magnitude lower rate than the charge transfer rates. This indicates that exciton dissociation via tunneling is the dominant pathway in the regime of short interdot distance and high coupling energy (b) whereas resonant energy transfer is dominant in low inter-NC coupling regime (c). All calculated transfer rates have error bars smaller than the symbols. [From Choi et al., Nano Lett. 10, 1805 (2010)].
(Color online) Illustration of confined-but-connected structures. (a) Colloidal NQD coated with organic ligands are strongly confined but isolated from one another. Complete coupling of the NQDs leads to nonfunctional sintered polycrystalline structures (b). The ideal case—a confined-but-connected structure that balances quantum confinement and coupling is illustrated in (c). (d) Tomographic reconstruction of a laser annealed PbSe NQD film and (e) cross-sectional annular dark field scanning transmission electron microscopy image of a PbSe/a-Si nanocomposite film annealed by a laser pulse. [From Baumgardner et al., ACS Nano 9, 7010 (2011)].
(Color online) Pressure-driven transformation of NQD assemblies. PbS nanosheets were synthesized by deviatoric stress-driven orientation and attachment of colloidal NQD. [From Wang et al., J. Am. Chem. Soc. 133, 14484 (2011)].
(Color online) Controlling the structure of self-assembled NQD superlattices. (a) GISAXS patterns of PbX NQD assembled into fcc, bcc, and bct superlattices. The model shows how the crystal symmetries are related through the Bain deformation. The wide-angle scattering (top right) shows the orientational ordering of PbS NQD in the lattice sites of the bcc superlattice. (b) Illustration of the relationship between particle shape and superlattice symmetry. Particles with an isotropic interaction potential behave as spheres and assemble into fcc superlattice; cuboctahedra particles exhibit anisotropic interactions and assemble into bcc superlattice with orientational ordering of the particles in their lattice sites. [From Bian et al., ACS Nano 5, 2815 (2011).]
Charge transport through NQD thin films. (a) Low bias (20 mV) conductance of PbSe NQD films shows temperature-dependent signatures consistent with variable range hopping [from Wehrenberg et al., J. Phys. Chem. B 109, 20192 (2005)]. (b) Schematic of a field-effect transistor test structure with NQD film as the channel device, (c) temperature dependence of field-effect mobility for an n-channel device assembled from In2Se2 2-capped 3.9 nm CdSe NQD.
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