HD-KFM & AFM Characterization of 2D Materials

Mapping surface potential, work function, and conductivity in graphene, hBN, and MoS₂ at the nanoscale — from moiré superlattices to layer-by-layer electronic structure.

Graphene Layer Identification - hBN Moiré Pattern Imaging - MoS₂ Layer-Dependent Conductivity - Single-Pass KPFM - ResiScope™ - Resistance Mapping

Why 2D Materials Demand Nanoscale Electrical Characterization

The electronic properties of 2D materials — graphene, hexagonal boron nitride (hBN), transition metal dichalcogenides (TMDs) such as MoS₂ — are exquisitely sensitive to layer count. A single additional atomic layer can shift the work function by tens of millivolts, switch a semiconductor to a semimetal, or introduce entirely new quantum phenomena such as moiré-driven flat bands.

Conventional optical and transport measurements provide spatially averaged information. Kelvin Probe Force Microscopy (KPFM) resolves these effects at the nanometer scale, mapping surface potential and work function across individual flakes, domain boundaries, and twisted van der Waals heterostructures with the spatial resolution that the physics demands.

~100 nm moiré domains resolvedLayer-by-layer work function contrast10² – 10¹² Ω resistance range

HD-KFM surface potential map of graphene on SiO₂ substrate, resolving monolayer (1ML), bilayer (2ML), trilayer (3ML), and four-layer (4ML) regions through work function contrast. Nano-Observer AFM with HD-KFM III.

Application 1

Hexagonal Boron Nitride (hBN): Moiré Pattern Visualization

Folded or twisted hBN layers form moiré superlattices — periodic interference patterns arising from rotational misalignment between atomic planes. These superlattices create long-range modulations in local electronic structure and are central to engineered quantum materials. HD-KFM maps the charge and potential modulations of moiré domains with the resolution needed to characterize them.

Multi-Scale HD-KFM Analysis of Folded hBN

From domain architecture to moiré superlattice — captured in a single characterization session

The Nano-Observer AFM with HD-KFM III provides multi-scale characterization of folded hBN across three complementary length scales, revealing the complete hierarchy of structural and electronic organization:

Scale	What is mapped	Key information
5 µm scan	Large-scale domain structure	Overall sample architecture, folded region geometry, domain size distribution
2 µm scan	Mid-range domain boundary mapping	Boundary sharpness, transition widths, lateral potential gradients between domains
1 µm scan	High-resolution moiré visualization	~100 nm triangular moiré domains, intralayer charge modulation, twist-angle dependent periodicity

Multi-scale HD-KFM surface potential maps of folded hBN. At 1 µm resolution, triangular moiré domains of approximately 100 nm are clearly resolved through periodic surface potential modulation. Nano-Observer AFM with HD-KFM III module.

Why single-pass HD-KFM is essential for moiré characterization

Moiré patterns in hBN produce surface potential modulations on the 10–100 mV scale across domains of ~100 nm. Conventional dual-pass KPFM degrades lateral resolution through tip-sample distance drift between topography and KFM passes, blurring domain boundaries and underestimating potential amplitude. HD-KFM's single-pass heterodyne detection eliminates this artifact, preserving the spatial resolution and potential contrast necessary to resolve individual moiré unit cells.

Application 2

Graphene: Layer-by-Layer Work Function Mapping

Graphene's electronic properties depend critically on layer count. Monolayer graphene is a zero-gap semimetal with linear Dirac dispersion; bilayer graphene has a parabolic band structure and a tunable bandgap; trilayer and thicker films shift toward graphite-like behavior. Identifying and characterizing these regions spatially is a fundamental requirement for 2D materials research and device fabrication.

Layer Identification by Surface Potential Contrast

Simultaneous topography and work function mapping across multi-layer graphene flakes

The Nano-Observer AFM with HD-KFM provides unprecedented layer identification capability through correlated topography and work function imaging. Each additional graphene layer produces a measurable, systematic shift in local surface potential, creating distinct color contrast in the KFM map that enables unambiguous layer assignment.

Region	Layer count	Work function behavior	HD-KFM map appearance
1ML	Monolayer	Lowest work function, strongest substrate influence (SiO₂ gating)	Distinct bright or dark tone; maximum potential contrast vs. substrate
2ML	Bilayer	Intermediate work function; reduced substrate screening	Intermediate tone; sharp boundary from 1ML region clearly visible
3ML	Trilayer	Approaching bulk-like value; Bernal vs. twisted stacking distinguishable	Progressive shift; stacking-order contrast detectable
4ML	Four-layer	Near-graphite work function; substrate effect screened	Saturated contrast plateau; distinct from thinner regions

Correlated topography (left) and HD-KFM surface potential map (right) of multi-layer graphene. Layer regions 1ML through 4ML are unambiguously distinguished by work function contrast, with sharp, well-defined interfaces between layer counts visible in both channels. Nano-Observer AFM with HD-KFM III.

The measurement simultaneously delivers: precise monolayer identification from the surface potential map, work function differences across the sample with intuitive color contrast, sharp, well-defined interfaces between regions with different layer counts, and direct correlation of electronic structure with topographic layer thickness.

Application 3

MoS₂: Combined HD-KFM and ResiScope™ Characterization

Molybdenum disulfide (MoS₂) is a transition metal dichalcogenide (TMD) with a dramatic layer-dependent electronic transition: bulk MoS₂ is an indirect bandgap semiconductor (1.2 eV), while monolayer MoS₂ becomes a direct bandgap semiconductor (1.8 eV) with strong photoluminescence. This transition is accompanied by measurable changes in both surface potential and electrical conductivity — both of which are accessible simultaneously with the Nano-Observer AFM.

Synergistic Electrical Characterization: HD-KFM + ResiScope™

Work function and resistance mapping on the same sample area — layer by layer

HD-KFM — Work Function

• Maps work function shifts with nanometer-scale resolution

• Reveals charge distribution across different layer counts

• Shows systematic surface potential changes with layer thickness

• Detects interlayer charge transfer at domain boundaries

ResiScope™ — Resistance

• Maps resistance across 10² to 10¹² Ω in a single scan

• Distinguishes conductive and insulating regions clearly

• Quantifies exponential conductivity change with layer count

• No manual gain adjustment — full range captured automatically

Correlated topography, HD-KFM surface potential, and ResiScope™ resistance maps of few-layer MoS₂, with accompanying quantitative line profiles. The resistance map reveals exponential conductivity changes across layer boundaries — data inaccessible to either technique alone. Nano-Observer AFM.

The combination of HD-KFM and ResiScope™ is uniquely powerful for TMD characterization because the two measurements are complementary and non-redundant. Work function (measured by HD-KFM) reflects changes in the electrostatic potential and band alignment; resistance (measured by ResiScope™) reflects carrier density and mobility. Together, they provide a complete picture of how the semiconductor-to-semimetal transition in MoS₂ evolves at the single-layer level.

ResiScope™ dynamic range advantage for MoS₂

MoS₂ samples routinely present resistance variations spanning many orders of magnitude within a single scan area — from highly conductive few-layer regions (10²–10⁴ Ω) to near-insulating monolayer domains on resistive substrates (10⁹–10¹² Ω). ResiScope™'s 10-decade dynamic range captures this full span in a single pass without gain switching, delivering quantitative resistance maps that conventional C-AFM amplifiers cannot obtain.

Technique

HD-KFM: Single-Pass Kelvin Force Microscopy

CSInstruments' HD-KFM (Heterodyne Detection Kelvin Force Microscopy) is the core enabling technology for high-resolution surface potential mapping in 2D materials research.

<10 nm Lateral resolution — surface potential

1 pass Single-pass — simultaneous topo + KFM

10¹⁰ Ω range ResiScope™ resistance dynamic range

~100 nm Moiré domain resolution in hBN

Single-pass vs. dual-pass KPFM for 2D materials

Standard dual-pass (lift mode) KPFM measures topography and surface potential in two sequential passes at different tip-sample separations. The additional lift distance used in the KFM pass reduces electrostatic signal strength and lateral resolution. For 2D materials, where domain features of 50–200 nm and potential modulations of 10–100 mV are typical, this resolution loss is often critical. HD-KFM's single-pass heterodyne approach maintains the tip at optimal distance throughout, delivering quantitatively more accurate work function values and sharper boundary definition.

→ HD-KFM III Module→ ResiScope III Module→ Nano-Observer II AFM→ KPFM Mode Overview

FAQ

Frequently Asked Questions

• Why use KPFM for 2D material characterization?

  KPFM (Kelvin Probe Force Microscopy) is the primary AFM technique for characterizing the electronic properties of 2D materials such as graphene, hBN, and MoS₂. It directly maps surface potential and work function at nanometer resolution, revealing layer-dependent electronic structure, moiré superlattice charge modulation, and interfacial charge transfer effects that are invisible to topography imaging alone. In single-pass HD-KFM mode, the technique achieves sub-10 nm spatial resolution with minimal electrostatic tip-sample perturbation, making it ideal for ultrathin van der Waals heterostructures.

• How does AFM reveal moiré patterns in hBN?

  Moiré patterns in folded or twisted hBN arise from periodic lattice interference between misaligned layers, creating superlattice modulations on the 10–100 nm scale. HD-KFM surface potential mapping on the Nano-Observer AFM resolves these features at multiple scales: large-scale domain structure (5 µm) showing overall architecture, mid-range mapping (2 µm) of domain boundaries, and high-resolution moiré visualization with ~100 nm triangular domains at 1 µm scale. The surface potential contrast directly reflects local variations in interlayer coupling and charge redistribution across the moiré unit cell.

• Can AFM distinguish graphene monolayer from bilayer and trilayer?

  Yes. HD-KFM on the Nano-Observer AFM distinguishes monolayer (1ML), bilayer (2ML), trilayer (3ML), and four-layer (4ML) graphene regions through work function contrast in the surface potential map. Each additional graphene layer shifts the local work function, producing distinct, reproducible color contrast in the KFM image. The technique simultaneously maps sharp, well-defined interfaces between regions with different layer counts and correlates electronic structure directly with topographic layer thickness measurements.

• How is MoS₂ characterized by AFM?

  MoS₂ characterization on the Nano-Observer AFM combines HD-KFM and ResiScope™ for comprehensive layer-dependent analysis. HD-KFM maps work function shifts with nanometer-scale resolution and reveals systematic changes in surface potential with layer thickness. ResiScope™ simultaneously maps resistance across 10 decades (10² to 10¹² Ω), clearly distinguishing conductive and insulating regions and quantifying how conductivity changes exponentially with the number of MoS₂ layers. This dual electrical characterization provides complementary insight into the semiconductor-to-semimetal transition in few-layer MoS₂.

• What is HD-KFM and how does it differ from standard KPFM?

  HD-KFM (Heterodyne Detection Kelvin Force Microscopy) is CSInstruments' proprietary single-pass KPFM implementation. Unlike conventional dual-pass (lift mode) KPFM, HD-KFM measures topography and surface potential simultaneously in a single scan, eliminating tip-sample distance drift artifacts that degrade KFM resolution in two-pass measurements. The heterodyne detection approach decouples the electrostatic signal at a higher harmonic, achieving sub-10 nm lateral resolution. For 2D materials research, this means sharper domain boundary contrast, more accurate work function quantification, and faster data acquisition compared to standard KPFM.

• What AFM is best for graphene and 2D materials research?

  The Nano-Observer AFM with HD-KFM III module is optimized for 2D materials characterization. Its single-pass heterodyne KFM implementation provides the spatial resolution necessary to map moiré superlattices, layer boundaries, and interlayer charge transfer in van der Waals heterostructures. The optional ResiScope™ module extends characterization to resistance mapping across 10 decades, enabling correlated work function and conductivity measurements on the same sample area — critical for understanding the layer-dependent electronic properties of semiconducting TMDs such as MoS₂, WS₂, and WSe₂.