Robert W. Meulenberg

Determination of the Exciton Binding Energy in CdSe Quantum Dots

The exciton binding energy (EBE) in CdSe quantum dots (QDs) has been determined using X-ray spectroscopy. Using X-ray absorption and photoemission spectroscopy, the conduction band (CB) and valence band (VB) edge shifts as a function of particle size have been determined and combined to obtain the true band gap of the QDs (i.e., without an exciton). These values can be compared to the excitonic gap obtained using optical spectroscopy to determine the EBE. The experimental EBE results are compared with theoretical calculations on the EBE and show excellent agreement.

Evidence for Ligand-Induced Paramagnetism in CdSe Quantum Dots

We report evidence that paramagnetism in CdSe QDs can be induced via manipulation of the surface chemistry. Using SQUID magnetometry and X-ray absorption spectroscopy, we demonstrate that the paramagnetic behavior of the CdSe QDs can be varied by changing the ligand end-group functionality of the passivating layer. Contrary to previous reports, no evidence for ferromagnetism was observed. The results suggest that the paramagnetism is induced via pi back-bonding between Cd 4d orbtials and ligands with empty pi* orbitals.

Ligand-mediated modification of the electronic structure of CdSe quantum dots.

X-ray absorption spectroscopy and ab initio modeling of the experimental spectra have been used to investigate the effects of surface passivation on the unoccupied electronic states of CdSe quantum dots (QDs). Significant differences are observed in the unoccupied electronic structure of the CdSe QDs, which are shown to arise from variations in specific ligand-surface bonding interactions.

Evidence of a ZnCr2Se4 spinel inclusion at the core of a Cr-doped ZnSe quantum dot.

Herein we report doping of ZnSe by Cr ions leads to formation of small ZnCr2Se4 spinel inclusions within the cubic sphalerite lattice of a 2.8 nm CrZnSe quantum dot (QD). The Cr ion incorporates as a pair of Cr(III) ions occupying edge-sharing tetragonal distorted octahedral sites generated by formation of three Zn ion vacancies in the sphalerite lattice in order to charge compensate the QD. The site is analogous to the formation of a subunit of the ZnCr2Se4 spinel phase known to form as inclusions during peritectoid crystal growth in the ternary CrZnSe solid-state compound. The oxidation state and site symmetry of the Cr ion is confirmed by X-ray absorption near edge spectroscopy (XANES), crystal field absorption spectroscopy, and electron paramagnetic resonance (EPR). Incorporation as the Cr(III) oxidation state is consistent with the thermodynamic preference for Cr to occupy an octahedral site within a II−VI semiconductor lattice with a half-filled t2g d-level. The measured crystal field splitting energy for the CrZnSe QD is 2.08 eV (2.07 eV form XANES), consistent with a spinel inclusion. Further evidence of a spinel inclusion is provided by analysis of the magnetic data, where antiferromagnetic (AFM) exchange, a Curie−Weiss (C−W) temperature of θ = −125 K, and a nearest-neighbor exchange coupling constant of JNN = −12.5 K are observed. The formation of stable spinel inclusions in a QD has not been previously reported. ■ INTRODUCTION The ability to incorporate paramagnetic ions into a semiconductor quantum dot (QD) lattice is of great interest to the solid-state materials community due to the ability to couple s− p−d orbital exchange-induced magnetic properties with the size-dependent QD energy levels. The excitement of incorporating a magnetic ion into an otherwise nonmagnetic host lattice reflects the potential of such materials for quantum computing, spintronics, magneto-resistive, and magneto-optical applications. While chemists are intrigued by the ability to control the location and concentration of the dopant ions as a crystallite grows, physicists are interested in the novel spin properties that arise and the influence of QD surfaces. For years it was considered nearly impossible to consistently dope a II−VI lyothermally grown QD due to selfannealing effects. Over the past several years the realization that a single source cluster precursor can be used to generate a predoped nucleus has led to the opportunity to efficiently incorporate a range of dopant ions into the whole II−VI series. In II−VI semiconductors, incorporation of a dopant metal ion with the same charge (i.e., Mn(II), Cr(II), and Co(II)) leads to simple substitutional incorporation of the ion of interest up to its solubility limit. Incorporation of an ion that has a different charge, such Cu(I) or Eu(III), results in formation of ion vacancies in the QD in order to reach charge neutrality. One magnetic dopant ion that has received little attention in the QD community is chromium. In Cr-doped II−VI semiconductors, the incorporation of Cr(I), Cr(II), and Cr(III) have been reported by solid-state synthesis. Of the three oxidation states, the Cr(III) oxidation state is preferred due to its one-half-filled t2g level and has been shown to be naturally introduced into octahedral (Oh) sites formed by ion vacancies that charge compensate the lattice in the otherwise tetrahedral coordination environment observed in II−VI semiconductors (ZnS, ZnSe, CdS, CdSe). Synthesis of the Cr(III)-doped II−VI semiconductor occurs via a peritectoid decomposition reaction leading to formation of a AB2X4 spinel structure by the group II metal occupying the tetrahedral (Td) site (A site) and the Cr(III) ion occupying the Oh site (B site). 28 The spinel phase is a singularity in the phase Received: November 1, 2011 Published: March 1, 2012 ArticleHerein we report doping of ZnSe by Cr ions leads to formation of small ZnCr(2)Se(4) spinel inclusions within the cubic sphalerite lattice of a 2.8 nm CrZnSe quantum dot (QD). The Cr ion incorporates as a pair of Cr(III) ions occupying edge-sharing tetragonal distorted octahedral sites generated by formation of three Zn ion vacancies in the sphalerite lattice in order to charge compensate the QD. The site is analogous to the formation of a subunit of the ZnCr(2)Se(4) spinel phase known to form as inclusions during peritectoid crystal growth in the ternary CrZnSe solid-state compound. The oxidation state and site symmetry of the Cr ion is confirmed by X-ray absorption near edge spectroscopy (XANES), crystal field absorption spectroscopy, and electron paramagnetic resonance (EPR). Incorporation as the Cr(III) oxidation state is consistent with the thermodynamic preference for Cr to occupy an octahedral site within a II-VI semiconductor lattice with a half-filled t(2g) d-level. The measured crystal field splitting energy for the CrZnSe QD is 2.08 eV (2.07 eV form XANES), consistent with a spinel inclusion. Further evidence of a spinel inclusion is provided by analysis of the magnetic data, where antiferromagnetic (AFM) exchange, a Curie-Weiss (C-W) temperature of θ = -125 K, and a nearest-neighbor exchange coupling constant of J(NN) = -12.5 K are observed. The formation of stable spinel inclusions in a QD has not been previously reported.

Modification of the conduction band edge energy via hybridization in quantum dots

X-ray absorption near edge structure spectroscopy (XANES) and theoretical modeling have been used to examine effects of hybridization on the conduction band edge in doped CdSe quantum dots (QDs). Experimentally, Cd M3-edge XANES provides evidence for a lowering of the CB minimum for Cu doped CdSe QDs that is dependent on Cu concentration. Theoretical modeling suggests the effects of hybridization between Cu and Cd atoms in the QD can explain our experimental results. The model can be extended for other dopant systems and provide a simple, yet effective, method to predict the effects of hybridization on the CB levels in QDs.

A Closer Look into the Traditional Purification Process of CdSe Semiconductor Quantum Dots

This paper describes how the postprocessing procedure for wurtzite CdSe quantum dots (QDs) 4.8 and 6.7 nm in diameter is affected by both the choice of nonsolvent and the number of processing steps. Using a host of analytical techniques (ultraviolet-visible, photoluminescence, nuclear magnetic, X-ray photoelectron, and infrared spectroscopy, as well as thermogravimetric analysis), we find that control over the ligand type and surface density can be achieved simply by the number of washing steps used during the postprocessing procedure. Using multiple washing steps we can achieve colloidally stable solutions of QDs with organic mass fractions as low as 13% by mass. For CdSe QDs passivated with trioctylphosphine oxide (TOPO) and stearic acid (SA), essentially no TOPO is bound to the particle surface after three or four washing steps, with a plateau in the amount of SA being removed. The results can be explained using the L- and X-type ligand classification system for QDs, with L-type ligands (TOPO) removed in the early processing steps but the removal of X-type (SA) ligand stalling at a large number of washing steps due to charging of the QDs. Importantly, very little change is observed in the photoluminescence (PL) properties, suggesting that the choice of nonsolvent during postprocessing will allow the production of QD materials with very low organic content by mass but with good PL quantum yields.

Synthesis and Characterization of Phase-Pure Manganese(II) and Manganese(III) Silicalite-2

Manganese silicalite-2 was synthesized at high pH using the molecular cluster Mn 12O 12(O 2CCH 3) 16 as a Mn source. The silicalite-2 (ZSM-11) materials were synthesized using 3,5-dimethyl- N, N-diethylpiperdinium hydroxide as a structure-directing agent to produce phase-pure ZSM-11 materials. No precipitation of manganese hydroxide was observed, and synthesis resulted in the incorporation of up to 2.5 mol % Mn into the silicalite-2 with direct substitution into the framework verified by the linear relationship between the unit cell volume and loading. The Mn is reduced to Mn (II) during hydrothermal synthesis and incorporated into the silicalite-2 framework during calcination at 500 degrees C. Further calcination at 750 degrees C does not affect the crystallinity but oxidizes essentially all of the Mn (II) to Mn (III) in the framework. The large difference in oxidation temperatures between the II and III oxidation states provides a means of producing relatively pure manganese(II) and manganese(III) silicalite-2 materials for applications such as catalysis.

Preparation of iron and gold silicide nanodomains on silicon (111) by the reaction of gold, iron-gold core-shell, and alloy nanoparticles with triethylsilane.

This study describes a strategy to use composite colloidal nanoparticles and triethylsilane as precursors to synthesize nanometer size structures on single-crystal silicon substrate. The concept is demonstrated by depositing gold, iron-gold alloy, and iron-gold core-shell nanoparticles on silicon (111). Upon heating, the nanoparticles form new crystalline phases on the Si (111) surface. Atomic force microscope (AFM) data show the collapse of the iron gold core-shell and alloy nanoparticles at temperatures 100-200 degrees C higher than gold nanoparticles, indicating the efficient tethering of iron containing nanoparticles on silicon (111). Both structural analysis and X-ray spectroscopy show that the iron-gold alloy and iron-gold core-shell nanoparticles successfully form the semiconducting beta-FeSi(2) phase at relatively low temperature. The stabilities of the silicide are assessed at elevated temperatures. Silicon successfully nucleates on the created nanostructures, which suggests strong catalytic activity towards producing further nanostructures on the surface.

Electronic structure of cobalt doped CdSe quantum dots using soft X-ray spectroscopy

The electronic structure and magnetic properties of cobalt doped CdSe quantum dots (QDs) are studied using electron microscopy, soft X-ray spectroscopy, and magnetometry. Magnetometry measurements suggest these QDs are superparamagnetic, contrary to a spin-glass state observed in the bulk analogue. Electron microscopy shows well formed QDs, but with cobalt existing as doped into the QD and as unreacted species not contained in the QD. X-ray absorption measurements at the Co L3-edge suggest that changes in spectra features as a function of particle size can be described considering combination of a cobalt ion in a tetrahedral crystal field and an octahedrally coordinated (impurity) phase. With decreasing particle sizes, the impurity phase increases, suggesting that small QDs can be difficult to dope.

X-ray absorption spectroscopy for the structural investigation of self-assembled-monolayer-directed mineralization.

Self-assembled monolayers (SAMs) of organothiol molecules prepared on noble metal substrates are known to exert considerable influence over biomineral nucleation and growth and, as such, offer model templates for investigation of the processes of directed biomineralization. Identifying the structural evolution of SAM/crystal systems is essential for a more comprehensive understanding of the mechanisms by which organic monolayers mediate mineral growth. X-ray absorption spectroscopy (XAS) provides the attractive ability to study SAM structure at critical stages throughout the processes of crystallization in SAM/mineral systems. Here, we discuss important theoretical and experimental considerations for designing and implementing XAS studies of SAM/mineral systems.