Showing papers by "Christopher B. Murray published in 2011"
TL;DR: A facile ligand-exchange approach is reported, which enables sequential surface functionalization and phase transfer of colloidal NCs while preserving the NC size and shape and represents an important step toward controllably engineering the surface properties of NCs.
Abstract: The ability to engineer surface properties of nanocrystals (NCs) is important for various applications, as many of the physical and chemical properties of nanoscale materials are strongly affected by the surface chemistry. Here, we report a facile ligand-exchange approach, which enables sequential surface functionalization and phase transfer of colloidal NCs while preserving the NC size and shape. Nitrosonium tetrafluoroborate (NOBF4) is used to replace the original organic ligands attached to the NC surface, stabilizing the NCs in various polar, hydrophilic media such as N,N-dimethylformamide for years, with no observed aggregation or precipitation. This approach is applicable to various NCs (metal oxides, metals, semiconductors, and dielectrics) of different sizes and shapes. The hydrophilic NCs obtained can subsequently be further functionalized using a variety of capping molecules, imparting different surface functionalization to NCs depending on the molecules employed. Our work provides a versatile l...
755 citations
TL;DR: These studies unambiguously demonstrate the abilities of facet-selective binding peptides in determining nanocrystal shape, representing a critical step forward in the use of biomolecules for programmable synthesis of nanostructures.
Abstract: The properties of a nanocrystal are heavily influenced by its shape. Shape control of a colloidal nanocrystal is believed to be a kinetic process, with high-energy facets growing faster then vanishing, leading to nanocrystals enclosed by low-energy facets. Identifying a surfactant that can specifically bind to a particular crystal facet is critical, but has proved challenging to date. Biomolecules have exquisite specific molecular recognition properties that can be explored for the precise engineering of nanostructured materials. Here, we report the use of facet-specific peptide sequences as regulating agents for the predictable synthesis of platinum nanocrystals with selectively exposed crystal surfaces and particular shapes. The formation of platinum nanocubes and nanotetrahedrons are demonstrated with Pt-{100} and Pt-{111} binding peptides, respectively. Our studies unambiguously demonstrate the abilities of facet-selective binding peptides in determining nanocrystal shape, representing a critical step forward in the use of biomolecules for programmable synthesis of nanostructures.
375 citations
TL;DR: The short, thiocyanate ligand gives rise to significantly enhanced electronic coupling between NCs as evidenced by large bathochromic shifts in the absorption spectra of CdSe and CdTe NC thin films and by conductivities as high as (2 ± 0.7) × 10(3) Ω(-1) cm(-1), allowing the first Hall measurements of nonsintered NC solids.
Abstract: Ammonium thiocyanate (NH4SCN) is introduced to exchange the long, insulating ligands used in colloidal nanocrystal (NC) synthesis. The short, air-stable, environmentally benign thiocyanate ligand electrostatically stabilizes a variety of semiconductor and metallic NCs in polar solvents, allowing solution-based deposition of NCs into thin-film NC solids. NH4SCN is also effective in replacing ligands on NCs after their assembly into the solid state. The spectroscopic properties of this ligand provide unprecedented insight into the chemical and electronic nature of the surface of the NCs. Spectra indicate that the thiocyanate binds to metal sites on the NC surface and is sensitive to atom type and NC surface charge. The short, thiocyanate ligand gives rise to significantly enhanced electronic coupling between NCs as evidenced by large bathochromic shifts in the absorption spectra of CdSe and CdTe NC thin films and by conductivities as high as (2 ± 0.7) × 103 Ω–1 cm–1 for Au NC thin films deposited from solut...
332 citations
TL;DR: This work demonstrates the first solution-processable metal-semiconductor nanocomposites with enhanced thermoelectric properties via carrier energy filtering with the possibility of combining a diverse set of n- and p-type semiconductor matrices with nanocrystals to engineer and optimize energy-dependent carrier scattering with the ease of materials processing.
Abstract: This work demonstrates the first solution-processable metal–semiconductor nanocomposites with enhanced thermoelectric properties via carrier energy filtering. Platinum nanocrystals are embedded in a p-type antimony(III) telluride (Sb2Te3) semiconductor matrix, thus introducing band-bending potentials for holes. By scattering low energy holes, an increase in thermopower is observed. Introduction of Pt nanocrystals also increases carrier concentration thereby partially compensating for reduced electrical conductivity due to the decreased mobility. At room temperature, an improvement in thermoelectric power factor was achieved compared to that of the Sb2Te3 films. This work highlights the possibility of combining a diverse set of n- and p-type semiconductor matrices with nanocrystals to engineer and optimize energy-dependent carrier scattering with the ease of materials processing.
214 citations
TL;DR: Thiocyanate as a ligand for lead sulfide nanocubes for high-performance, thin-film electronics enables the fabrication of CMOS-like inverters with promising gains of ∼22 from a single semiconductor material.
Abstract: We report the use of thiocyanate as a ligand for lead sulfide (PbS) nanocubes for high-performance, thin-film electronics. PbS nanocubes, self-assembled into thin films and capped with the thiocyanate, exhibit ambipolar characteristics in field-effect transistors. The nearly balanced, high mobilities for electrons and holes enable the fabrication of CMOS-like inverters with promising gains of ∼22 from a single semiconductor material. The mild chemical treatment and low-temperature processing conditions are compatible with plastic substrates, allowing the realization of flexible, nonsintered quantum dot circuits.
189 citations
TL;DR: A simple, robust solution-phase synthesis of ultrathin ceria nanoplates in the presence of mineralizers is reported, which can be easily controlled by changing reaction parameters, such as precursor ratio, reaction time, etc.
Abstract: Shape-controlled synthesis of inorganic nanomaterials has received great attention due to their unique shapedependent properties and their various applications in catalysis, electronics, magnetics, optics, and biomedicine. Among these nanomaterials, ultrathin twodimensional (2D) anisotropic nanomaterials are especially attractive due to their high surface-to-volume ratio and potential quantum size effects. A variety of approaches have been developed to prepare such nanomaterials. Typical methods include vapor deposition, templated synthesis, electrochemical deposition, sol–gel processing, and solvothermal/hydrothermal treatments. Solution-phase chemical synthesis has proven particularly effective in controlling the size and morphology of the nanomaterials. Ceria has been widely used in catalysis, optics, sensors, and solid oxide fuel cells. Due to its high oxygen storage capacity (OSC), which originates from easy conversion between CeO2 and CeO2 x, ceria has found its primary utilization in catalysis as an oxygen carrier. Ceria nanomaterials with various morphologies, mainly polyhedra, have been reported. Recently, 1D ceria nanostructures, such as nanowires, have also been reported. However, with the exception of one report on the preparation of nanosheets, well-controlled 2D ceria nanomaterials have not been explored and the comparison of the OSC properties between 3D and 2D structures has not been possible. On the other hand, the different properties of the (100), (110), and (111) ceria facets has been debated. There is no consensus on whether crystallographic orientation or particle size affects reactivities. Therefore, high-quality ceria nanocrystals selectively exposing different low Miller-index surfaces, are crucial to enabling experiments that resolve the controversy. Here we report a simple, robust solution-phase synthesis of ultrathin ceria nanoplates in the presence of mineralizers. The morphology of nanoplates can be easily controlled by changing reaction parameters, such as precursor ratio, reaction time, etc. In addition, we also prepare ceria nanomaterials in various 3D morphologies by hydrothermal and combustion methods. The OSC of our 2D ceria materials have been tested and compared to the OSC of their 3D counterparts. In brief, the synthesis of ceria nanoplates involves the thermal decomposition of cerium acetate at 320–330 8C in the presence of oleic acid and oleylamine as stabilizers and employs sodium diphosphate or sodium oleate as mineralizers. Transmission electron microscopy (TEM) images of ceria nanoplates are shown in Figure 1. Square ceria nanoplates (S-nanoplates, Figure 1a) with an edge length of 11.9 nm (s= 7%), are synthesized with sodium diphosphate as the mineralizer while elongated ceria nanoplates (Lnanoplates, Figure 1e) with a length of 151.6 nm (s= 9%) and a width of 14.3 nm (s= 12%), are produced with sodium oleate as the mineralizer. The nanoplates in both samples have a thickness of about 2 nm. As shown in Figure 1c and g, the stacks of nanoplates confirm that the sample consists of 2D plates rather than 3D cubes or rods. S-nanoplates readily form the demonstrated stacking arrays as seen in drop-cast TEM samples. L-nanoplates only form stacks by a selfassembly at a liquid–liquid (e.g. hexane–ethylene glycol) interface. The S-nanoplates also self-assemble to a ceria nanosheet at a hexane–acetonitrile interface, as shown in Figure 3a. High-resolution TEM (HRTEM) images of both nanoplates (Figures 1d,h and S1c in the Supporting Information) reveal an interplanar distance of 0.27 nm, consistent with the (200) lattice spacing of the ceria crystal. The fast Fourier transform (FFT) patterns confirm the {100} textures of ceria nanoplates. Plates (e.g. square plates) could be enclosed by either six (100) facets or a combination of two (100) facets and four (110) facets. As illustrated in Figure S1, our HRTEM images and simulations of HRTEM images suggest that our ceria nanoplates are enclosed by six (100) [*] D. Y. Wang, Y. J. Kang, Prof. C. B. Murray Department of Chemistry University of Pennsylvania, Philadelphia, PA 19104 (USA) E-mail: cbmurray@sas.upenn.edu
172 citations
TL;DR: This work reports the growth of large-area (∼ 1 cm(2), transferable BNSL monolayers using the liquid-air interfacial assembly approach and demonstrates theLiquid- air interfacialAssembly of BNSl bilayers which exhibit unique superlattice structures that have not been observed in the 3D B NSLs.
Abstract: The modular assembly of multicomponent nanocrystal (NC) superlattices enables new metamaterials with programmable properties. While self-assembly of three-dimensional (3D) binary NC superlattices (BNSLs) has advanced significantly in the past decade, limited progress has been made to grow 2D BNSLs such as monolayers and bilayers over extended areas. Here, we report the growth of large-area (∼ 1 cm(2)), transferable BNSL monolayers using the liquid-air interfacial assembly approach. The BNSL monolayers are formed by an entropy-driven assembly process with structures tunable by varying the NC size ratio. We further demonstrate the liquid-air interfacial assembly of BNSL bilayers which exhibit unique superlattice structures that have not been observed in the 3D BNSLs. As a further extension, bilayered ternary NC superlattices (TNSLs) are obtained by the cocrystallization of three types of NCs at the liquid-air interface.
162 citations
TL;DR: The identification and the ability to tune the relative phase stability of polymorphic structures provide a unique opportunity to engineer the interparticle coupling through controlled clustering and/or interconnectivity of sublattice in BNSLs with identical stoichiometry.
Abstract: We report the formation and systematic struc-tural characterization of a new AB6 polymorph with the body-centered cubic (bcc) symmetry in binary nanocrystal superlattices (BNSLs). The bcc-AB6 phase...
87 citations
TL;DR: The directed liquid interfacial assembly of gadolinium trifluoride (GdF(3) nanoplates into liquid crystalline assemblies displaying long-range orientational and positional order is reported.
Abstract: Directed self-assembly of colloidal nanocrystals into ordered superlattices enables the preparation of novel metamaterials with diverse functionalities. Structural control and precise characterization of these superlattices allow the interactions between individual nanocrystal building blocks and the origin of their collective properties to be understood. Here, we report the directed liquid interfacial assembly of gadolinium trifluoride (GdF3) nanoplates into liquid crystalline assemblies displaying long-range orientational and positional order. The macroscopic orientation of superlattices is controlled by changing the subphases upon which liquid interfacial assembly occurs. The assembled structures are characterized by a combination of transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) measurements performed on a laboratory diffractometer. By doping GdF3 nanoplates with europium (Eu3+), luminescent phosphorescent superlattices with controlled structure are produced and enable ...
85 citations
TL;DR: This work reports an unusual yet universal approach to rapidly assembling hierarchically organized NC films that display highly periodic, tunable microscale stripe patterns over square centimeter areas while preserving the local superlattice structure.
Abstract: Self-assembly of nanocrystals (NCs) into periodically ordered structures on multiple length scales and over large areas is crucial to the manufacture of NC-based devices. Here, we report an unusual yet universal approach to rapidly assembling hierarchically organized NC films that display highly periodic, tunable microscale stripe patterns over square centimeter areas while preserving the local superlattice structure. Our approach is based on a drying-driven dynamic assembly process occurring on a liquid surface with the stripe pattern formed by a new type of contact-line instability. Periodic ordering of NCs is realized on microscopic and nanoscopic scales simultaneously without the need of any specialized equipment or the application of external fields. The striped NC superlattice films obtained can be readily transferred to arbitrary substrates for device fabrication. The periodic structure imparts interesting modulation and anisotropy to the properties of such striped NC assemblies. This assembly approach is applicable to NCs with a variety of compositions, sizes, and shapes, offering a robust, inexpensive route for large-scale periodic patterning of NCs.
77 citations
TL;DR: This work performs temperature-dependent thermopower measurements on PbTe nanocrystal solids to directly probe the position of E(F) - E(T) and observes that as the size of the nanocrystals reduces, E (F)- E( T) increases primarily due to the widening of density of state (DOS) gap.
Abstract: The position of the Fermi energy level (E(F)) with respect to the energy level where the transport process occurs (transport energy level, E(T)) is an important parameter that determines the electrical properties of semiconductors. However, little attention has been devoted to investigating the position of E(F) in semiconductor nanocrystal solids, both theoretically and experimentally. In this study, we perform temperature-dependent thermopower measurements on PbTe nanocrystal solids to directly probe E(F) - E(T). We observe that as the size of the nanocrystals reduces, E(F) - E(T) increases primarily due to the widening of density of state (DOS) gap. Furthermore, by modifying the monodispersity of nanocrystals, we observe an increase in thermopower as the distribution of energy states sharpens. This work promotes a deeper understanding of thermal occupation of energy states as well as electronic transport processes in semiconductor nanocrystal solid systems.
TL;DR: This work reports the growth of NaCl-type binary nanocrystal (NC) superlattice membranes by coassembly of FePt and MnO NCs at the liquid-air interface, opening a new route to the fabrication of ordered ferromagnetic NC arrays on a desired substrate for high-density data storage applications.
Abstract: We report the growth of NaCl-type binary nanocrystal (NC) superlattice membranes by coassembly of FePt and MnO NCs at the liquid–air interface. The constituent FePt NCs were converted into the hard magnetic L10 phase by thermal annealing at 650 °C without degradation of the long-range NC ordering. In contrast, both FePt-only NC superlattices and FePt–MnO disordered NC mixtures showed substantial FePt sintering under the same annealing conditions. Our results demonstrate that the incorporation of FePt NCs into binary superlattices can solve the problems of FePt sintering during conversion to the L10 phase, opening a new route to the fabrication of ordered ferromagnetic NC arrays on a desired substrate for high-density data storage applications.
TL;DR: The wavelength of the resonance is within the window of transparency for biological tissue making it a potential in vivo temperature marker or transducer for in situ heating with deep-penetrating near-infrared radiation as mentioned in this paper.
Abstract: Monodisperse 3.1 nm Ag2Te NCs show a sharp optical absorption feature at 1154 nm with a large linear temperature dependence and the photoconductive response consistent with semiconducting character in Ag2Te NCs. The wavelength of the resonance is within the window of transparency for biological tissue making it a potential in vivo temperature marker or transducer for in situ heating with deep-penetrating near-infrared radiation.
TL;DR: In this paper, the role of different precursors for PbSe nanorod (NR) synthesis was studied, focusing on phosphine chemistry to understand precursor decomposition.
Abstract: We studied the role of different Se precursors for PbSe nanorod (NR) synthesis, focusing on phosphine chemistry to understand precursor decomposition. After characterizing the morphology of PbSe na...
TL;DR: By applying surface modification to n- and p-dope Pb Se NW FETs, the first PbSe NW inverters are fabricated, demonstrating the promise of these nanostructured materials in integrated circuits.
Abstract: Wet-chemical methods, under rigorous air-free conditions, were used to synthesize single-crystalline 10 nm diameter PbSe nanowires (NWs), and electric-field, directed assembly was employed to align NW arrays to form the semiconducting channels of field-effect transistors (FETs). Electrical measurements revealed as-aligned NWs in bottom, gold, contact FETs are predominantly p-type ambipolar, consistent with the presentation of small barriers to electron and hole injection for this low band gap semiconductor. Exposing the NW FET to UV-ozone p-doped the NWs, illustrating the sensitivity of PbSe to oxygen, but controlled oxidation allowed the fabrication of unipolar p-type FETs. Selectively exposing the contact region of as-aligned NW FETs to low to moderate concentrations of hydrazine, commonly used to n-dope nanocrystal and NW devices, switched the predominantly p- to n-type ambipolar behavior as if the entire NW channel was exposed. At these hydrazine concentrations, charge transfer doping the metal-semiconductor interface dominates the FET characteristics. Only upon exposing the NW FETs to high hydrazine concentrations did charge transfer doping of the NW channel overcome the large intrinsic, thermally generated carrier concentration of this low band gap material, modulating the NW carrier concentration and forming unipolar n-type FETs. Pulling low vacuum removed surface hydrazine returning the predominantly p-type ambipolar FET behavior. Doping and dedoping with hydrazine were repeatedly reversible. By applying surface modification to n- and p-dope PbSe NW FETs, we fabricated the first PbSe NW inverters, demonstrating the promise of these nanostructured materials in integrated circuits.
Journal Article•
TL;DR: Good agreement between the measured and calculated spectra indicates that resonances in the local field factors underlie the measured spectra of PbSe nanocrystals and nanorods.
Patent•
03 Oct 2011TL;DR: In this paper, the monodisperse particles of the invention have a rare earth-containing lattice which in one embodiment may be an yttrium containing lattice or in another may be a lanthanide-containing one.
Abstract: Monodisperse particles having: a single pure crystalline phase of a rare earth-containing lattice, a uniform three-dimensional size, and a uniform polyhedral morphology are disclosed. Due to their uniform size and shape, the monodisperse particles self assemble into superlattices. The particles may be luminescent particles such as down-converting phosphor particles and up-converting phosphors. The monodisperse particles of the invention have a rare earth-containing lattice which in one embodiment may be an yttrium-containing lattice or in another may be a lanthanide-containing lattice. The monodisperse particles may have different optical properties based on their composition, their size, and/or their morphology (or shape). Also disclosed is a combination of at least two types of monodisperse particles, where each type is a plurality of monodisperse particles having a single pure crystalline phase of a rare earth-containing lattice, a uniform three-dimensional size, and a uniform polyhedral morphology; and where the types of monodisperse particles differ from one another by composition, by size, or by morphology. In a preferred embodiment, the types of monodisperse particles have the same composition but different morphologies. Methods of making and methods of using the monodisperse particles are disclosed.
TL;DR: Analysis of the population dynamics of multiple excitons in PbS quantum dots using spectrally resolved ultrafast supercontinuum transient absorption measurements shows that bleaching associated with biexciton population is red shifted with respect to the single exciton feature, which is in accordance with a positive binding energy for the bieexciton.
Abstract: We examine the multiple exciton population dynamics in PbS quantum dots by ultrafast spectrally-resolved supercontinuum transient absorption (SC-TA). We simultaneously probe the first three excitonic transitions over a broad spectral range. Transient spectra show the presence of first order bleach of absorption for the 1S_h-1S_e transition and second order bleach along with photoinduced absorption band for 1P_h-1P_e transition. We also report evidence of the one-photon forbidden 1S_{h,e}-1P_{h,e} transition. We examine signatures of carrier multiplication (multiexcitons for the single absorbed photon) from analysis of the first and second order bleaches, in the limit of low absorbed photon numbers ( ~ 10^-2), at pump energies from two to four times the semiconductor band gap. The multiexciton generation efficiency is discussed both in terms of a broadband global fit and the ratio between early- to long-time transient absorption signals.. Analysis of population dynamics shows that the bleach peak due to the biexciton population is red-shifted respect the single exciton one, indicating a positive binding energy.