Leif O. Brown

High-Resolution Spectral Analysis of Individual SERS-Active Nanoparticles in Flow

Nanoparticle spectroscopic tags based on surface enhanced Raman scattering (SERS) are playing an increasingly important role in bioassay and imaging applications. The ability to rapidly characterize large populations of such tags spectroscopically in a high-throughput flow-based platform will open new areas for their application and provide new tools for advancing their development. We demonstrate here a high-resolution spectral flow cytometer capable of acquiring Raman spectra of individual SERS-tags at flow rates of hundreds of particles per second, while maintaining the spectral resolution required to make full use of the detailed information encoded in the Raman signature for advanced multiplexing needs. The approach allows multiple optical parameters to be acquired simultaneously over thousands of individual nanoparticle tags. Characteristics such as tag size, brightness, and spectral uniformity are correlated on a per-particle basis. The tags evaluated here display highly uniform spectral signatures, but with greater variability in brightness. Subpopulations in the SERS response, not apparent in ensemble measurements, are also shown to exist. Relating tag variability to synthesis parameters makes flow-based spectral characterization a powerful tool for advancing particle development through its ability to provide rapid feedback on strategies aimed at constraining desired tag properties. Evidence for single-tag signal saturation at high excitation power densities is also shown, suggesting a role for high-throughput investigation of fundamental properties of the SERS tags as well.

Room-temperature Coulomb-blockade-dominated transport in gold nanocluster structures

In this paper we discuss the near-room-temperature electrical transport characteristics of structures made from ligand-stabilized metal clusters. The structures show threshold behaviour, nonlinear current-voltage characteristics and radio-frequency-induced plateaux consistent with Coulomb-blockade-dominated transport in disordered arrays of metal dots. Samples having triphenylphosphine and octadecanethiol ligand shells are found to have a 3 orders of magnitude difference in current above threshold. We discuss a possible explanation for this observation.

Coulomb-blockade dominated transport in patterned gold-cluster structures

In this paper we present the fabrication and near-room temperature electrical transport properties of structures made from the gold-cluster material Au55[P(C6H5)3]12Cl6. We discuss the use of electron-beam lithography to define the structures laterally and compare the direct current-voltage characteristics of non-patterned and patterned structures. In both cases non-linear behavior is observed with features that are consistent with Coulomb blockade dominated transport in disordered arrays of clusters. Radio frequency induced plateaus in the current-voltage characteristics demonstrate coherent tunneling. Finally, we show that other ligand stabilized gold-cluster materials can be used to form ordered gold-cluster arrays.

Fabrication and near-room temperature transport of patterned gold cluster structures

Ligand stabilized metal clusters are becoming of considerable interest for possible nanoscale electronics applications. In this article, we report the fabrication and near-room temperature electrical transport properties of structures made from the gold-cluster material Au55[P(C6H5)3]12Cl6. While other strategies to produce cluster arrays have been reported, this work is the first to use electron-beam lithography to laterally define the structures. We compare the current–voltage characteristics of nonpatterned and patterned structures, and show that in both cases the nonlinear behavior observed is consistent with Coulomb blockade dominated transport. We argue that charging of individual Au55 cores is responsible for the effects observed.

Fabrication and electrical transport characteristics of low-dimensional nanoparticle arrays organized by biomolecular scaffolds

We report the use of a biopolymer scaffold in the nanofabrication of low-dimensional arrays of gold nanoparticles. The room-temperature current-voltage dependence shows threshold behavior characteristic of Coulomb-blockade. Above threshold the current varies linearly with voltage which suggests one-dimensional behavior. Capacitance estimates are consistent with transport through a disordered chain containing a minimum of 200 nanoparticles.

Room Temperature Single Electron Charging in Gold Nanoparticle Networks Formed on Biopolymer Templates

We report the current-voltage characteristics of gold nanoparticle – biopolymer networks at room temperature. Above a threshold voltage the current-voltage relationship is almost linear. From the current-voltage scaling above threshold we argue that one-dimensional regions of the network dominate the transport. Periodic features in the conductance are found in many samples. Both the threshold voltage and the conductance features occur at voltages much greater than expected for the capacitance of the nanoparticles. Possible explanations for the structure are considered.