Scanning Probe Microscopy Platform

NREL has greatly enhanced the utility of the scanning electron microscope (SEM) by combining it with scanning probe microscopy (SPM) in a scanning probe microscopy platform.

We have integrated an SPM system inside an electron microscope to create a platform of new capabilities. These capabilities allow better-controlled manipulation of nanostructures or selection of an area for high-precision observation. This platform is compatible with a helium closed-circuit cryostat and fully accessible to the optics of cathodoluminescence (CL) detectors.

The scanning probe platform is based on a nanopositioning system with closed-feedback electronics for higher accuracy. An assembly of high thermal conductivity copper braids coupled to a low-vibration closed-circuit cryostat provides variable temperature (50-300 K). Modes of operation include scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Force sensors are based on self-sensing and self-actuating piezoelectric tuning forks.

Modes of Operation

We have developed four innovative modes of operation combining SPM and electron microscopy.

Scanning Tunneling Luminescence 

Scanning tunneling luminescence (STL) assesses nanostructures. Based on STM, luminescence is stimulated by recombination of tunneling electrons. In conductors, the observed emission is caused by deexcitation of surface plasmons (SPs) — collective oscillations of free electrons. In semiconductors, photons are emitted by recombination of electrons with available holes or vice versa (unipolar conditions) or by impact ionization (bipolar conditions), depending on the voltage applied to the tip. Photon maps with unprecedented resolution have been obtained when synchronizing the scanning of the tip with the detection of the luminescence. As a proof of principle, we have resolved individual quantum dots of 2-5 nm in diameter by using STL.

Lateral Transport

Lateral transport is used for investigating grain boundaries. Based on a combination of STM and SEM, electrons (holes) excited by the electron beam at specific locations are detected through the STM tip allowing lateral electron diffusion to be measured.

Electroluminescence Mapping

Electroluminescence (EL) mapping assesses diodes. Based on conductive atomic force microscopy, EL applies an external bias to cause individually injected current pulses to be visible during intermittent contact of the oscillating and conducting tip. It follows that EL can be stimulated by this current during the contact cycle. Maps of the EL and current can be acquired simultaneously. Acquisition of the local L-V characteristic and EL spectrum are possible.

Near-Field Cathodoluminescence

Near-field cathodoluminescence is designed to be a surface-sensitive CL based on near-field scanning optical microscopy. The luminescence is excited by the electron beam and detected through the optical fiber positioned in the near field.

Scanning Probe Microscopy Platform Capability Examples

Left: Scanning probe techniques support surface science at the atomic and molecular level; this image obtained using a scanning tunneling microscope shows gray and white clusters of microscopic terraces in a sample gold substrate of a semiconductor device. Right: Scanning probe techniques produce high-resolution color images or maps like this one obtained using scanning tunneling luminescence and showing microscopic surface plasmons in a sample device; they appear as small brown, yellow, and gray clusters.

Gold substrate: (Left) STM image reveals the terraces of the H2 flamed substrate. (Right) Corresponding STL map of the SP emission. The color represents the contribution of the localized (red) and propagating (green) SPs to the overall STL.

Left: Spectrum of high-resolution image of gold substrate in a semiconductor device sample; the image was produced using scanning tunneling luminescence and shows surface plasmons in the optical cavity formed by the tunneling gap. Right: A schematic of the optical cavity, measured at high resolution by scanning tunneling luminescence, of a sample gold substrate in a semiconductor device.

Gold substrate: (Left) Scanning tunneling luminescence spectrum shows SP modes (n=1,2,3) in the optical cavity formed by the tunneling gap. (Right) Schematics of the optical cavity sustaining the SP modes.

Schematic and graphical sample image of three-dimensional yellow and red clusters representing the tuning-fork sensor in atomic force microscopy.

Schematics of the tuning-fork (TF) sensor for AFM. The TF is driven by an oscillating voltage source near its resonance frequency, and the cantilever oscillates normally to the surface. The interaction between the tip and the surface causes a reduction of the oscillation amplitude and a shift in the resonance frequency and phase of the oscillation.

High-resolution atomic force microscopic image of a sample copper gallium diselenide solar cell, showing red and yellow clusters. Electroluminescence map of the sample copper gallium diselenide solar cell shown at left; the map image shows individual grains as widely separated flat-looking yellow clusters on a maroon background.

CuGaSe2 solar cell. AFM image (left) and corresponding electroluminescence map (right) revealing individual grains of higher efficiency.

Graph showing measured output current, in millivolts per millisecond, of tuning fork sensor in atomic force microscopy.

Highly reproducible current pulses have been obtained in tapping mode operation.