Material junctions form the basis of modern solid-state technology by controlling current flow and are key components in digital electronics, electronic switches, signal amplification and processing, sensing, light-emitting diodes, lasers, and photovoltaics. A heterojunction arises when dissimilar semiconductors come into contact and form an abrupt interface. The junction physics are controlled by the band gap, electron affinity, and chemical potential or Fermi level of each material as well as how the energy levels or bands align across the material interface (which can be influenced by dipoles or interfacial defects/alloying). Over the years, engineering the material interface and film characteristics of thin-film heterojunctions has resulted in the development of many advanced devices.

Nanoscale and quantum-confined materials offer new physics for optical and optoelectronic devices and have inspired advanced synthetic methods to create multicomponent nanostructures containing material junctions by selectively arranging individual domains for complex functionalities. Some examples of heterostructuring in nanomaterials include photoluminescence manipulation in core–shell quantum dots (QDs), slowed cooling, reduced blinking, materials with plasmon-assisted absorption enhancement and doping, charge-separating interfaces, energy funneling, and strain effects. Such nanoheterostructures offer promising new materials for modernizing industries such as biological sensing, photovoltaics, and photocatalysis.

Progress in the synthesis of nanocrystals and nanoheterostructures has led to the manipulation of particle size, shape, morphology, and surface chemistry. However, with such small structures and domains, the ability to accurately characterize many aspects of the individual components and their interfaces is greatly reduced compared to macroscopic bulk and thin film materials. Since the objects are so small (far below diffraction limits), weak signals arise that are often at or below the noise level of most instrumentation. Therefore, averaging large numbers of nanocrystals becomes the logical way to achieve manageable signal-to-noise ratios for many techniques. While such “ensemble”-type measurements provide useful physical/chemical information about a collection of nanostructures, characterizing the difference between inhomogeneous and homogeneous effects (such as size or shape dispersion vs intrinsic homogeneous broadened line widths) is critical for developing optimal structures. Interfaces govern electronic properties of the overall heterostructure and affect operation; therefore, the ability to understand individual nanoheterostructures will greatly assist the design of novel complex materials.

One unique instrument that is capable of measuring nanoscale surface properties with high spatial resolution is the scanning probe microscope (SPM). Since the advent of scanning tunneling microscopy (STM) and atomic force microscopy (AFM), our understanding of surface science and nanotechnology has improved dramatically. These techniques have helped researchers understand surface structure, adsorbed molecules, tethered metal clusters, semiconductor nanocrystals (NCs), and biological molecules with nanometer-scale resolution. Both techniques have the same rudimentary design consisting of a tip (probe) attached to a piezoelectric tube or motor that permits precise control over the tip position, raster scanning across and interacting with a surface. However, the type of interaction between the tip and the surface that is monitored and used as feedback differentiates these two techniques. In this review, we will describe the two scanning probe techniques briefly and discuss experimental methods that helped elucidate electrical characterization of single-component and heterostructured nanomaterials. Typically, for scanning probe experiments, NCs are assembled onto conducting substrates by solution-phase casting techniques such as spin-, drop-, or dip-coating. Additionally, NCs could also be grown directly on a sample surface or tethered by use of bifunctionalized molecules. The density of particles on the surface allows one to study either individual single-particle physics or meso/macroscopic effects of electronic coupling when the nanomaterials are densely packed. Both SPM techniques will be crucial for advancing the development of heterostructured nanomaterials toward new technologies. We will review various effects arising from quantum confinement and nanoscopic interfaces that have been studied by both STM and AFM.

http://pubs.acs.org/doi/full/10.1021/cr500280t