Colin M. Hessel

Copper Selenide Nanocrystals for Photothermal Therapy

Ligand-stabilized copper selenide (Cu(2-x)Se) nanocrystals, approximately 16 nm in diameter, were synthesized by a colloidal hot injection method and coated with amphiphilic polymer. The nanocrystals readily disperse in water and exhibit strong near-infrared (NIR) optical absorption with a high molar extinction coefficient of 7.7 × 10(7) cm(-1) M(-1) at 980 nm. When excited with 800 nm light, the Cu(2-x)Se nanocrystals produce significant photothermal heating with a photothermal transduction efficiency of 22%, comparable to nanorods and nanoshells of gold (Au). In vitro photothermal heating of Cu(2-x)Se nanocrystals in the presence of human colorectal cancer cell (HCT-116) led to cell destruction after 5 min of laser irradiation at 33 W/cm(2), demonstrating the viabilitiy of Cu(2-x)Se nanocrystals for photothermal therapy applications.

Alkyl passivation and amphiphilic polymer coating of silicon nanocrystals for diagnostic imaging.

A method to produce biocompatible polymer-coated silicon nanocrystals for medical imaging is shown. Silica-embedded Si nanocrystals are formed by HSQ thermolysis. The nanocrystals are then liberated from the oxide and terminated with Si-H bonds by HF etching, followed by alkyl monolayer passivation by thermal hydrosilylation. The Si nanocrystals have an average diameter of 2.1 nm ± 0.6 nm and photoluminesce with a peak emission wavelength of 650 nm, which lies within the transmission window of 650-900 nm that is useful for biological imaging. The hydrophobic Si nanocrystals are then coated with an amphiphilic polymer for dispersion in aqueous media with the pH ranging between 7 and 10 and an ionic strength between 30 mM and 2 M, while maintaining a bright and stable photoluminescence and a hydrodynamic radius of only 20 nm. Fluorescence imaging of polymer-coated Si nanocrystals in biological tissue is demonstrated, showing the potential for in vivo imaging.

Synthesis and Photoluminescent Properties of Size-Controlled Germanium Nanocrystals from Phenyl Trichlorogermane-Derived Polymers

We report the preparation of luminescent oxide-embedded germanium nanocrystals (Ge-NC/GeO2) by the reductive thermal processing of polymers derived from phenyl trichlorogermane (PTG, C6H5GeCl3). Sol-gel processing of PTG yields air-stable polymers with a Ge:O ratio of 1:1.5, (C6H5GeO1.5)n, that thermally decompose to yield a germanium rich oxide (GRO) network. Thermal disproportionation of the GRO results in nucleation and initial growth of oxide-embedded Ge-NC, and subsequent reaction of the GeO2 matrix with the reducing atmosphere results in additional nanocrystal growth. This synthetic method affords quantitative yields of composite powders in large quantities and allows for Ge-NC size control through variations of the peak thermal processing temperature and reaction time. Freestanding germanium nanocrystals (FS-Ge-NC) are readily liberated from Ge-NC/GeO2 composite powders by straightfoward dissolution of the oxide matrix in warm water. Composites and FS-Ge-NC were characterized using thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX), and photoluminescence (PL) spectroscopy.

Raman Spectroscopy of Oxide-Embedded and Ligand-Stabilized Silicon Nanocrystals

Oxide-embedded and oxide-free alkyl-terminated silicon (Si) nanocrystals with diameters ranging from 3 nm to greater than 10 nm were studied by Raman spectroscopy. For ligand-passivated nanocrystals, the zone center Raman-active mode of diamond cubic Si shifted to lower frequency with decreasing size, accompanied by asymmetric peak broadening, as extensively reported in the literature. The size dependence of the Raman peak shifts, however, was significantly more pronounced than previously reported or predicted by the RWL (Richter, Wang, and Ley) and bond polarizability models. In contrast, Raman peak shifts for oxide-embedded nanocrystals were significantly less pronounced as a result of the stress induced by the matrix.

Room temperature hydrosilylation of silicon nanocrystals with bifunctional terminal alkenes.

H-terminated Si nanocrystals undergo room temperature hydrosilylation with bifunctional alkenes with distal polar moieties-ethyl ester, methyl ester, or carboxylic acids-without the aid of light or added catalyst. The passivated Si nanocrystals exhibit bright photoluminescence (PL) and disperse in polar solvents, including water. We propose a reaction mechanism in which ester or carboxylic acid groups facilitate direct nucleophilic attack of the highly curved Si surface of the nanocrystals by the alkene.

Colloidal Silicon Nanorod Synthesis

The colloidal synthesis of crystalline silicon (Si) nanorods with diameters of 5 to 10 nm and lengths of 15 to 75 nm is demonstrated. Trisilane was decomposed in a hot solvent in the presence of dodecylamine and gold (Au) nanocrystals. Nanorods form by Au-seeded solution-liquid-solid growth with dodecylamine serving as capping ligands that stabilize the nanorod dispersion. Post-synthesis etching of the Au seeds from the nanorod tips is also demonstrated.

Colloidal Luminescent Silicon Nanorods

Silicon nanorods are grown by trisilane decomposition in hot squalane in the presence of tin (Sn) nanocrystals and dodecylamine. Sn induces solution-liquid-solid nanorod growth with dodecylamine serving as a stabilizing ligand. As-prepared nanorods do not luminesce, but etching with hydrofluoric acid to remove residual surface oxide followed by thermal hydrosilylation with 1-octadecene induces bright photoluminescence with quantum yields of 4-5%. X-ray photoelectron spectroscopy shows that the ligands prevent surface oxidation for months when stored in air.

Ordered Structure Rearrangements in Heated Gold Nanocrystal Superlattices

Small-angle X-ray scattering (SAXS) data reveal that superlattices of organic ligand-stabilized gold (Au) nanocrystals can undergo a series of ordered structure transitions at elevated temperature. An example is presented of a body-centered cubic superlattice that evolves into a hexagonal close-packed structure, followed by the formation of binary simple cubic AB13 and hexagonal AB5 superlattices. Ultimately the superlattice decomposes at high temperature to bicontinuous domains of coalesced Au and intervening hydrocarbon. Transmission electron microscopy revealed that the ordered structure transformations result from partial ligand desorption and controlled Au nanocrystal growth during heating, which forces changes in superlattice symmetry. These observations suggest some similarity between organic ligand-coated nanocrystals and microphase-segregated diblock copolymers, where thermally induced nanophase-segregation of Au and organic ligand influences the ordered arrangements in the superlattice.

Silicon Nanocrystal Superlattices

Colloidal nanocrystals with precisely controlled size and shape can be assembled into ordered superlattices.[1–3] Superlattices have been made of a wide range of materials, from semiconductors to metals to insulators, and have been explored for various applications, including sensors,[4–6] transistors,[7,8] data storage,[9] solar cells,[10] and thermoelectrics.[11] Nonetheless, colloidal nanocrystal superlattices have not yet been made of one of the most commercially relevant semiconductors, silicon (Si). Here, we report the first colloidal silicon (Si) nanocrystal superlattices. Additionally, we examined their thermal stability and found that they are much more robust than other types of nanocrystal superlattices, retaining their structural order to relatively high temperatures (>350°C) because of the strong covalent bonding of the hydrocarbon capping layer. Si nanocrystals are interesting for many applications, but especially for optoelectronics requiring light emission. Bulk Si is a poor light emitter due to its indirect band gap, but Si nanocrystals—or quantum dots—can exhibit bright, size-tunable, visible photoluminescence and electroluminescence,[12] making them suitable as a down-converting phosphor or active emitting material in light-emitting diodes.[13] Si quantum dots are even being explored as a laser source.[14] Photovoltaic devices utilizing Si quantum dots have also been proposed[15] because of their size-tunable optical absorption edge, much higher absorption coefficient and the potential for multiexciton generation (MEG).[16] Most of these applications require arrays of nanocrystals and there have been efforts to fabricate them. But the collections of Si nanocrystals studied to date have been disordered, usually with a significant size distribution. Some success towards order has been achieved by thermally annealing alternating layers of amorphous Si and SiO2 to yield periodically stacked monolayers of relatively monodisperse Si nanocrystals in SiO2, but without positional in-plane order.[17–19] Here, we report the self-assembly of colloidal Si nanocrystal superlattices with face centered cubic (fcc) order. Si nanocrystals were synthesized by thermal decomposition of hydrogen silsesquioxane (HSQ) followed with HF etching, thermal hydrosilylation with 1-dodecene and finally a size-selective precipitation. The nanocrystals were dispersed in chloroform and drop cast. Ordering of the nanocrystals was observed by both transmission electron microscopy (TEM) and grazing incidence small angle X-ray scattering (GISAXS), as shown in Figure 1. The GISAXS pattern exhibits both rings and spots that index to an fcc superlattice with a lattice constant of aSL=15.5 nm. The occurrence of diffraction spots indicates specific orientations of superlattice domains with respect to the substrate. The spots index to fcc superlattice domains with (111)SL and (100)SL planes parallel to the substrate (i.e., (111) and (100)-oriented). TEM images (as in Figure 1), showed regions of fcc superlattice oriented with (111)SL and (112)SL planes parallel to the substrate. Based on the sizes of the diffraction spots in the GISAXS patterns, these ordered superlattice grains are about 120 nm in diameter (See Supporting Information). The occurrence of the scattering rings in the GISAXS pattern indicates that there are superlattice grains randomly oriented with respect to the substrate as well (See Supporting Information). Figure 1 A) GISAXS pattern from a Si nanocrystal superlattice. The pattern indexes to a FCC suplerattice structure. The circles and squares highlight spots associated specific orientations of superlattice domains parallel to the substrate: (001)SL and (111)SL ... Based on the lattice constant of aSL=15.5 nm, the nearest neighbour interparticle separation (center-to-center) is 11.0 nm. The Si core diameter of the nanocrystals determined from SAXS measurements of solvent-dispersed nanocrystals was 8.0±1.2 nm (Supporting Information). The length of a fully-extended C12 alkyl chain is 1.7 nm, so the edge-to-edge separation of the nanocrystals (3.0 nm) is slightly less than the twice the length of the fully extended capping ligands (3.4 nm). Based on the volume of the superlattice occupied by ligand, there appears to be a slight excess of free ligand in the superlattice.[20] The thermal stability of the Si nanocrystal superlattices was also tested. Figure 2 shows GISAXS of a fcc superlattice of Si nanocrystals as it was heated from 35°C to 375°C. The (111) and (220) diffraction spots are still observed in the GISAXS pattern up to 280°C, although the higher order diffraction spots have disappeared, indicating that some disorder occurs but the superlattice retains its fcc structure. Even up to 375°C—the highest temperature possible in the experimental setup—the Si nanocrystal superlattice showed (111) and (220) diffraction spots, indicating that the nanocrystals remain unsintered. There was, however, a slight contraction in the lattice as it was heated above 280°C due to loss of ligand. Figure 2 A–E) GISAXS of a (100)-oriented FCC superlattice of Si nanocrystals as it was heated to the indicated temperature. F) TGA of Si nanocrystals. The Si nanocrystal superlattices are much more stable than superlattices of other kinds of nanocrystals. For example, superlattices of dodecanethiol-capped Ag nanocrystal disorder at 180°C.[21] Superlattices of oleic acid- capped PbSe nanocrystals,[22] oleic acid-capped PbS nanocrystals,[23] and dodecanethiol-capped Au nanocrystals[24] sinter at much lower temperatures of 168°C, 230°C, and 200°C, respectively. The high thermal stability is attributed to the strong covalent Si-C bonded alkane ligand layer on the Si nanocrystals, although it is also possible that partial oxidation of the Si nanocrystals surface during heating in air helps limit sintering. Thermal gravimetric analysis (TGA) of Si nanocrystals (Figure 2F) showed three stages of ligand desorption: (1) evaporation of free ligand at around 190°C; (2) desorption and evaporation of bound ligand between 270°C and 520°C; and (3) significant oxidation of the Si core between 520°C and 800°C. Figure 3 shows a TEM image of an fcc superlattice of smaller 2.4 nm diameter Si nanocrystals capped with a mixture of 1-dodecene and 1-octadecene. (More TEM images are provided in supporting information.) From TEM, the (110)SL d-spacing is 4.8 nm, corresponding to a superlattice lattice constant of aSL=8.3 nm. From SAXS of solution-dispersed nanocrystals, the Si core diameter is 2.4±0.52 nm. The polydispersity of just over 20% is somewhat surprising since this degree of polydispersity usually prevents ordering; however, there may be a size-selection and purification that occurs during the superlattice formation process. Based on the superlattice dimensions and the nanocrystal diameter, it is clear that there is excess free ligand in the superlattice. This was obvious when drop-casting the nanocrystals. Unlike the larger nanocrystals, the smaller nanocrystals could not be completely dried into a film, even when stored under vacuum at 150°C for 12 hours due to the excess ligand in the sample. As in the case of the larger diameter nanocrystals, excess ligand appears to be important for helping superlattice order, in this case easing the strain in the superlattice that would arise from the relative polydispersity of the nanocrystals. Figure 3 TEM image of an fcc superlattice of 2.4 nm diameter Si nanocrystals capped with a mixture of dodecene/octadene. Inset: Fast Fourier transform (FFT) of the TEM image indexed to an fcc superlattice. The superlattice is oriented with (111)SL planes parallel ... Colloidal Si nanocrystal superlattices were self-assembled and characterized by TEM and GISAXS. The superlattices exhibit fcc structure for both large (8.0 nm diameter) and small (2.4 nm diameter) nanocrystals. GISAXS showed that a significant amount of superlattice grains had specific orientations with respect to the substrate with (111)SL and (100)SL planes parallel to the underlying substrate. Superlattices with (112)SL planes oriented parallel to the substrate were also observed by TEM. The Si nanocrystal superlattices were found to be very thermally stable, much more than other types of nanocrystal superlattices. Their thermal stability appears to relate to the robust covalent Si-C bonding of the capping ligand layer.

Gold seed removal from the tips of silicon nanorods.

A chemical method was developed to remove the gold (Au) seed particles from the tips of solution-liquid-solid (SLS) grown silicon (Si) nanorods. The nanorods are capped with hydrophobic ligands during the synthesis, which made it necessary to perform the Au etching in an aqua regia and chloroform emulsion. Preliminary etching experiments revealed that a thin Si shell coated the Au seeds and prevented Au removal. Therefore, a rapid thermal quench of the reaction mixture was needed to crack this shell and provide etchant access to the Au seed. More than 95% of the Au seeds could be removed from the tips of thermally quenched samples without damaging the crystalline Si nanorods.