Detection of a Chirality-Induced Spin Selective Quantum Capacitance in α-Helical Peptides

Abstract

Advanced Kelvin probe force microscopy simultaneously detects the quantum capacitance and surface potential of an α-helical peptide monolayer. These indicators shift when either the magnetic polarization or the enantiomer is toggled. A model based on a triangular quantum well in thermal and chemical equilibrium and electron-electron interactions allows for calculating the electrical potential profile from the measured data. The combination of the model and the measurements shows that no global charge transport is required to produce effects attributed to the chirality-induced spin selectivity effect. These experimental findings support the theoretical model of Fransson et al. Nano Letters 2021, 21 (7), 3026-3032. Measurements of the quantum capacitance represent a new way to test and refine theoretical models used to explain strong spin polarization due to chirality-induced spin selectivity.

Keywords: Kelvin probe force microscopy; chirality-induced spin selectivity; quantum capacitance; spinterface.

Figure 1

Time-averaged FM-KPFM scans (b–d, f–h) of the contact potential difference U_CPD showing a Au-reference square surrounded by a peptide monolayer (a) at ω_m = 1.4 kHz. The hand and magnet symbols indicate the configurations (b) L-peptide with magnet pointing to south and (d) L-peptide with magnet pointing to north. (f) D-peptide with magnet pointing south and (h) D-peptide with magnet pointing north. The false color bars (c,g) display the fitted and normalized U_CPD distributions of b and d and f and g. The potential difference between the Au-reference area (blue peak) and the SAM (yellow peak) is ΔU = 165 ± 4 mV (b), 106 ± 4 mV (d), 117 ± 8 mV (f), and 190 ± 8 mV (h). Expected Kelvin parabola hysteresis (e) in south (gray) and north (red) configuration based on the voltage-clamp mechanism of the equivalent circuit in part a.

Figure 2

Modulation voltage amplitude U_AC and the average tip–sample distance d_t do not affect the surface potential difference U_CPD. Red data corresponds to a north and gray to a south magnetic polarization. Data in a and b are from the l-enantiomer, c and d from the d-enantiomer. The solid black line corresponds to the measured average U_CPD with FM-AFM/FM-KPFM in the scans of Figure 1 and the dashed to the confidence interval thereof.

Figure 3

Phase difference Δφ at a calculated distance d_t = 32 nm in the four different enantiomer and magnetization configurations with one curve each recorded on the Au reference and on peptides. The solid lines correspond to Δφ_LP, and the dashed lines correspond to Δφ_LP + Δφ_CISS. The insets of b shows the measured time-domain Kelvin parabola where blue data points correspond to U_↑ and red to U_↓.

Figure 4

Average tip–sample distance d_t dependent measurements of the dynamically induced potential shift ΔU_↑↓ (a,c) and the capacitance gradient difference ⟨d²C/dz²⟩_↑↓ (b,d). Dashed lines correspond to fits of the corresponding models. Red data correspond to a north and gray to a south magnetic polarization. Data in a and b are obtained from the l-enantiomer and c and d from the d-enantiomer.

Figure 5

Profile of the effective potentials for electrons. The distance is normalized to the monolayer thickness t. The gray dashed outline corresponds to situations where U_CPD ± 2V is applied to the tip, generating a field within the tip–monolayer gap. Gray lines correspond to a high potential, south configuration and red to a low potential, north configuration.