Scanning Photocurrent Microscopy Overview

Updated 10 July 2026

Scanning Photocurrent Microscopy (SPCM) is a laser-scanning technique that maps local photoresponse to analyze carrier dynamics in electronic materials.
It quantifies the interplay of photogeneration, carrier drift and diffusion, and device geometry to characterize junctions and barrier effects.
SPCM has been adapted into far-field, near-field, cryogenic, and ultrafast regimes, providing versatile insights into material properties.

Scanning photocurrent microscopy (SPCM) is a laser-scanning technique that spatially resolves photoresponse in electrically contacted devices by raster-scanning a focused optical beam and recording the resulting photocurrent or photovoltage as a function of position. In its standard implementation, the measured signal reflects the interplay of local photogeneration, carrier drift and diffusion, thermoelectric conversion, contact and junction electrostatics, and the electrode geometry that collects the induced current. Across one- and two-dimensional semiconductors, metallic layered compounds, correlated oxides, Weyl semimetals, and photovoltaic heterostructures, SPCM has been used to localize built-in fields, image p–n junction formation, extract transport lengths and lifetimes, distinguish competing photoresponse mechanisms, and extend photocurrent mapping into ultrafast, near-field, frequency-domain, and cryogenic structured-light regimes (Kasırga, 11 Sep 2025).

1. Measurement principle and observables

In far-field SPCM, a diffraction-limited spot is scanned across a device while the dc or lock-in-detected photocurrent is recorded, usually together with reflected light for spatial co-registration. The core observable is position-dependent current under local excitation, commonly at zero source–drain bias for built-in-field studies or at finite bias for photoconductive and bolometric response. Representative implementations include confocal microscopes with galvo-mirror beam scanning, piezo-stage rastering, or sample scanning, with spot sizes ranging from a few hundred nanometers to about a micrometer in visible-light microscopy; large-area LBIC-derived systems trade resolution for throughput, while cryogenic and near-field variants alter the spatial and spectral reach of the method (Reuter et al., 2017, Laird et al., 2022, Hao et al., 30 May 2025).

The measured signal is not a universal proxy for a single microscopic process. In conventional semiconductor devices, local maxima often coincide with Schottky barriers, depletion regions, heterojunctions, or gate-defined p–n junctions, where built-in fields efficiently separate photoexcited carriers. In gapless conductors and semimetals, however, the collected current can be a nonlocal projection of locally generated current onto the device weighting field. This distinction is formalized by the Shockley–Ramo relation,

$I = \int d^3\mathbf{r}\; \mathbf{J}(\mathbf{r}) \cdot \mathbf{E}_w(\mathbf{r}),$

or, in two-dimensional reductions, by overlap of the local photoinduced current with a geometry-dependent weighting field or weighting potential gradient. A recurrent implication is that SPCM maps can conflate current generation and current collection unless the local transport problem and the electrode geometry are interpreted together (Wang et al., 2022, Mayes et al., 2020).

Short-circuit photocurrent and open-circuit photovoltage are complementary rather than interchangeable observables. SPCM in the strict sense usually refers to short-circuit current mapping, whereas scanning photovoltage microscopy emphasizes open-circuit voltage weighting. Several studies explicitly exploit both viewpoints: junction-localized photocurrent identifies where photocarriers are separated, while photovoltage mapping can reveal how global device geometry channels local electromotive forces into measured signals (Mayes et al., 2020).

2. Instrumentation and operating configurations

A standard SPCM instrument comprises a focused optical source, a beam-positioning subsystem, synchronized electrical readout, and a co-registered optical imaging channel. The source may be a monochromatic laser, a tunable supercontinuum, or a modulated LED. Mechanical choppers, direct source modulation, and lock-in detection are common because they suppress background and extend sensitivity into the sub-nA or even pA regime. In low-dimensional devices, visible wavelengths such as 514.5 nm, 532 nm, 550–600 nm, and 633 nm recur frequently, but wavelength tuning into the near-IR or mid-IR is used when absorption thresholds, excitons, or broadband semimetallic responses are under study (Ubrig et al., 2014, Wu et al., 2013, Wang et al., 2019).

Representative laboratory realizations span widely different operating envelopes. A low-cost microscope built around a zoom-lens inspection system, a fiber-coupled green LED, motorized translation stages, and a sourcemeter achieved local photocurrent mapping on 2D-material devices for under $10{,}000$ EUR, with a measured spot full width at half maximum of $(2.3 \pm 0.3)\,\mu\mathrm{m}$ using a $10\,\mu\mathrm{m}$ core fiber (Reuter et al., 2017). At the opposite end, cryogenic scanning photocurrent spectrometry with structured light combined a supercontinuum source, an acousto-optic tunable filter, a spatial light modulator, and lock-in readout to perform spatially resolved spectroscopy from $500$ to $700\,\mathrm{nm}$ , over a $35 \times 25\,\mu\mathrm{m}^2$ field, with $\sim 1\,\mu\mathrm{m}$ spatial resolution, temperatures down to $3\,\mathrm{K}$ , and magnetic fields up to $\pm 14\,\mathrm{T}$ (Hao et al., 30 May 2025).

SPCM is not restricted to spatial imaging at a single wavelength. Scanning photocurrent spectroscopy acquires spectra at each position and is used to resolve excitonic resonances, band-edge features, and polarization or field-dependent selection rules. IMPS microscopy extends LBIC/SPCM into the frequency domain by measuring the complex small-signal photocurrent transfer function $10{,}000$ 0 at each pixel, thereby mapping diffusion, recombination, interfacial kinetics, and RC limitations across photovoltaic devices (Laird et al., 2022, Hao et al., 30 May 2025).

Advanced geometries add further control variables. Femtosecond SPCM combines spatial scanning with pump–probe delay to image carrier transit on the ps timescale. Near-field photocurrent nanoscopy replaces the far-field spot with a nano-optical tip that launches confined plasmons or phonon-polaritons. Scanning photocurrent tunneling microscopy replaces macroscopic contact collection with tunneling into a scanning tip, pushing spatial resolution from the diffraction limit to the sub-nanometer scale (Son et al., 2014, Rikhter et al., 2023, Li et al., 2023).

3. Mechanisms and governing models

A central methodological issue in SPCM is mechanism assignment. Similar-looking spatial maps can arise from distinct processes, and the dominant mechanism can change with bias, gate voltage, excitation level, temperature, substrate, or contact design.

Mechanism	Characteristic SPCM signature	Representative contexts
Photovoltaic	Peaks at Schottky barriers, depletion regions, p–n junctions	CNT p–n junctions, MoS $10{,}000$ 1, WS $10{,}000$ 2
Photothermoelectric	Zero-bias signals from Seebeck mismatch and $10{,}000$ 3	Metal/TMD contacts, VO $10{,}000$ 4, NbS $10{,}000$ 5
Bolometric	Bias-dependent conductance change over heated regions	Metallic 2D crystals, biased channels
Hot-carrier or non-equilibrium contributions	Zero-bias or spectral behavior unexplained by PV/PTE alone	1L–ML MoS $10{,}000$ 6 junctions
Symmetry-enabled nonlinear edge currents	Edge-localized response without engineered junctions	Td-WTe $10{,}000$ 7
Ambipolar high-injection transport	Exponential tails without Schottky contact	Lightly doped or intrinsic semiconductors