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. 2009 Apr;9(4):1451-6.
doi: 10.1021/nl803298q.

Ultrastable atomic force microscopy: atomic-scale stability and registration in ambient conditions

Gavin M King  1 , Ashley R Carter, Allison B Churnside, Louisa S Eberle, Thomas T Perkins
Affiliations

Ultrastable atomic force microscopy: atomic-scale stability and registration in ambient conditions

Gavin M King et al. Nano Lett. 2009 Apr.

Abstract

Instrumental drift in atomic force microscopy (AFM) remains a critical, largely unaddressed issue that limits tip-sample stability, registration, and the signal-to-noise ratio during imaging. By scattering a laser off the apex of a commercial AFM tip, we locally measured and thereby actively controlled its three-dimensional position above a sample surface to <40 pm (Deltaf = 0.01-10 Hz) in air at room temperature. With this enhanced stability, we overcame the traditional need to scan rapidly while imaging and achieved a 5-fold increase in the image signal-to-noise ratio. Finally, we demonstrated atomic-scale ( approximately 100 pm) tip-sample stability and registration over tens of minutes with a series of AFM images on transparent substrates. The stabilization technique requires low laser power (<1 mW), imparts a minimal perturbation upon the cantilever, and is independent of the tip-sample interaction. This work extends atomic-scale tip-sample control, previously restricted to cryogenic temperatures and ultrahigh vacuum, to a wide range of perturbative operating environments.

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Figures

Figure 1
Figure 1
Schematic of the experimental layout. (a) Detailed view of the tip and sample shows focused lasers scattering off an AFM tip and a fiducial mark (silicon disk) on the sample. Back-scattered signals were collected and used to deduce the position of the tip and the sample relative to each laser beam. (b) Two stabilized diode lasers (SDL) at different wavelengths [λ = 810 nm (green), λ = 845 nm (red)] were sent into the microscope and focused by a high numerical-aperture objective (Obj). Back-scattered light was separated from the incoming light by an optical isolator formed by a polarizing beam splitter (PBS) and a quarter-wave plate (λ/4). At different wavelengths, the signals were separated by dichroic mirrors and detected by independent QPDs. A third laser [λ = 785 nm (blue)] was reflected off the backside of the cantilever for force control during AFM imaging. Tip and sample control were achieved via feedback loops to two PZT stages. Blue-shaded components are in optically conjugate planes. The long axis of the cantilever was along the y-axis.
Figure 2
Figure 2
AFM tip detection in three dimensions. (a–c) Records of the QPD output vs tip position are shown for x (red), y (blue), and z (green) on the moving axis (solid lines) and the stationary axes (dashed lines) from light scattered off a commercial silicon tip. Traces offset vertically for clarity.
Figure 3
Figure 3
Picometer-scale AFM tip control in 3D at ambient conditions. (a) Tip position records vs time were low-pass filtered to 10 Hz and offset vertically for clarity [x (red), y (blue), z (green)]. Positions were determined by an “out-of-loop” monitor laser while the tip was actively stabilized with the other laser. (b) A scatter plot of the tip position in the x–z plane from the 100 s record in panel a. Histograms of the data projected onto the x and z axes were well fit by Gaussians with standard deviations of 28 and 26 pm, respectively.
Figure 4
Figure 4
Improved signal-to-noise ratio in an image. (a) Time record of the cantilever-imaging signal when the tip was engaged on the surface and held stable at a single pixel. Analysis of these records showed standard deviations of 0.47, 0.24, and 0.09 nm when low-pass filtered to 5 kHz (light purple), 500 Hz (orange), and 50 Hz (dark purple), respectively. (b–d) Sequential images ofa5nm gold nanosphere taken with increased averaging. Specifically, the averaging times per pixel were 0.2, 2, and 20 ms for panels b–d, respectively. (e) Line scans through the center region of images [b (light purple), c (orange), and d (dark purple)].
Figure 5
Figure 5
Ultrastable AFM imaging and residual drift analysis. (a–c) Images ofa5nm gold nanosphere taken at times T = 0, 41, and 82 min, respectively. (d) The 2D cross-correlation between the first and last images. (e) A 1D slice (dots) through the 2D cross-correlation is plotted with the corresponding Gaussian fit (red line). The nanosphere's location [xp = 25.199 ± 0.004 nm (peak ± σfit)] is indicated graphically (dashed line thickness = 5σfit). This 1D slice is centered vertically on the 2D cross-correlation peak (black dotted line in panel d). (f) Relative lateral position of the nanosphere plotted vs time as determined by cross-correlation analysis [x (red), y (blue)]. From linear fits to the data (lines), we deduced residual lateral drift rates of 4 and 5 pm/min in x and y, respectively.
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