Multiparametric Kelvin Probe Force Microscopy for the Simultaneous

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Multiparametric Kelvin Probe Force Microscopy for the Simultaneous Mapping of Surface Potential and Nanomechanical Properties Hui Xie,* Hao Zhang, Danish Hussain, Xianghe Meng, Jianmin Song, and Lining Sun The State Key Laboratory of Robotics and Systems, Harbin Institute of Technology, Harbin 150080, PR China

ABSTRACT: We report high-resolution multiparametric kelvin probe force microscopy (MP-KPFM) measurements for the simultaneous quantitative mapping of the contact potential difference (CPD) and nanomechanical properties of the sample in single-pass mode. This method combines functionalities of the force−distance-based atomic force microscopy and amplitudemodulation (AM) KPFM to perform measurements in single-pass mode. During the tip−sample approach-and-retract cycle, nanomechanical measurements are performed for the retract part of nanoindentation, and the CPD is measured by the lifted probe with a constant tip−sample distance. We compare the performance of the proposed method with the conventional KPFMs by mapping the CPD of multilayer graphene deposited on n-doped silicon, and the results demonstrate that MP-KPFM has comparable performance to AM-KPFM. In addition, the experimental results of a custom-fabricated polymer grating with heterogeneous surfaces validate the multiparametric imaging capability of the MP-KPFM. This method can have potential applications in finding the inherent link between nanomechanical properties and the surface potential of the materials, such as the quantification of the electromechanical response of the deformed piezoelectric materials.

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INTRODUCTION Knowledge of the surface potential and nanomechanical properties is becoming increasingly important in understanding the function of microelectronic devices, microbial activity, and numerous electromechanical and biological phenomena. For instance, the surface charge distribution is correlated to the performance of active devices1,2 and protein adsorption or cell adhesion.3 In addition, the mechanical properties of the material affect its electrical properties,4−6 and it will be affected by the structural parameters of devices.7 Similarly, the membrane potential and nanomechanical properties of cells play a critical role in cell behavior and mechanotransduction.8−10 The mechanical properties and surface potential have been extensively studied by independent methods. Atomic force microscopy (AFM) is a powerful tool for measuring the nanomechanical properties of materials, cells, and biomolecules. Initially, qualitative measurements of the nanomechanical properties were performed using the phase feedback signal in tapping mode (intermittent contact mode).11 The first quantitative mapping of the nanomechanical properties was carried out in force−volume mode.12 Pulsed-force13 and peak force modes14 were subsequently adopted for fast nanomechanical characterization with high precision. The ability of © XXXX American Chemical Society

AFM to measure nanomechanical properties has been widely exploited for cell mechanics,15,16 the nanomechanics of polymer/cellulose fibrils,17−20 and biomaterials.21,22 Kelvin probe force microscopy (KPFM) is a well-known method for measuring the nanoscale surface potential because of its high spatial resolution.23 It has been widely used to study memory devices,24 photovoltaic devices,25 corrosion science,26,27 the surface potential of biomolecules,28,29 electrical properties of the functional materials,30−34 dielectric materials,35 and triboelectric nanogenerators,36 to name a few. In conventional KPFM, topography and the contact potential difference (CPD) are measured in the double- or single-pass modes without nanomechanical characterization.23,37,38 The electrical properties or mechanical properties have also been studied by contact mode KPFM (cKPFM)39 and by photothermal excitation contact resonance AFM40 in contact mode. They are more suitable for hard samples because of the presence of lateral forces and the strong interaction forces between the probe and sample. Newly developed PeakForce KPFM can be used to map the topography, CPD, and Received: December 21, 2016 Revised: February 8, 2017 Published: March 6, 2017 A

DOI: 10.1021/acs.langmuir.6b04572 Langmuir XXXX, XXX, XXX−XXX

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Langmuir mechanical properties in double-pass mode.41 However, scanning the same sample area two times is time-consuming and prone to topographical errors caused by electrostatic forces.37,42 In many cases, the simultaneous measurement of the nanoscale mechanical and electrical properties can be highly significant, e.g., the study of the flexoelectric materials and mechanosensitive ion channels of the cells. However, their concurrent measurements have been a challenge. In this work, we report a high-resolution multiparametric Kelvin probe force microscopy (MP-KPFM) study that combines the functionalities of the force−distance (FD)-based AFM and amplitudemodulation (AM) KPFM to perform simultaneous quantitative mapping of the CPD and nanomechanical properties in singlepass mode. Scan results of multilayer graphene on n-doped silicon demonstrate that the CPD resolution of MP-KPFM and AM-KPFM is comparable. In addition, scanning a polymer grating via MP-KPFM shows its capability to simultaneously measure the topography, CPD, and nanomechanical properties such as elastic modulus. This method can have potential applications in the simultaneous mapping of nanoscale electrical and mechanical properties such as mechanotransduction mapping of the cells.

2πεh tan 2(θ ) dl h+z ⎛ h + z⎞ ⎟ = 2πε tan 2(θ )⎜h − z ln ⎝ z ⎠

C tip =

∫0

h

(4)

Similarly, the electrostatic force and its gradient due to the tip can be given as ⎛ h+z h ⎞ ⎟ Ftip = πεΔU 2 tan 2(θ )⎜ln − ⎝ z h + z⎠ ′ = −πεΔU 2 tan 2(θ ) Ftip

h2 z(h + z)2

(5)

(6)

From eqs 2, 3, 5, and 6, the contribution from the tip as a function of its integral height is simulated for both AM and FM modes. As shown in Figure 1, simulation results reveal that the

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NUMERICAL SIMULATION OF KPFM In KPFM, the probe−sample system behaves as a parallel plate capacitor, and the potential difference between them is given as ΔU = UDC + UAC sin(ωt) ± UCPD, where UDC + UAC sin(ωt) is the external potential and UCPD is CPD between the probe and sample. Numerous simulations of KPFM41,43,44 have been done, which are based on the parallel plate capacitor model. The KPFM control loop adjusts UDC to UCPD according to the amplitude (AM mode) or frequency feedback (FM mode) signal from a lock-in amplifier, and thus the electric force or electric force gradient is nullified at ω. To elaborate the contribution to the CPD from the tip (cone and apex) and cantilever in AM and FM modes, numerical simulations are performed. For simulations, a silicon probe is used that has a cantilever length (L), width (W), tip height (H), and half cone angle (θ) of 225 μm, 27.5 μm, 15 μm, and 20°, respectively. The probe is modeled as a rectangular beam with a cone-shaped tip that is assumed to be perpendicular to the sample surface. The equivalent capacitance at the free end of the cantilever can be given as Ccan =

∫0

L

εWl dl (H + z + (L − l)sin α)L

Figure 1. Contribution of the tip to the CPD as a function of the integral height of the tip. Curves are simulation results for different distances from the tip apex to the sample surface. Circles show the electric force, and triangles show the electric force gradient. The distances from the tip apex to the sample surface are 10, 30, 50, 80, and 100 nm, respectively.

contribution of the tip is highly distance-dependent: for increasing z, the tip contribution decreases for both modes. However, CPD in the AM mode is influenced by the longrange averaging character of the electrostatic force from the tip apex, the tip cone, and the cantilever. In FM mode, the local CPD between the tip apex and sample underneath can be measured to high resolution. Although an averaging effect exists in both modes, the lateral resolution of the AM mode is lower than that of the FM mode at the same tip−sample distance.

(1)

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where ε is the relative dielectric constant, l is the distance from the integral part of the cantilever to the fixed end of the cantilever, z is the distance from the tip apex to the sample surface, and α is the angle between the cantilever and sample surface (α = 7°). The electrostatic force due to the cantilever is given as Fcan =

−1 ∂Ccan (ΔU )2 2 ∂z

(2)

From eq 1 to eq 2, electrostatic force gradient can be given as ′ =− Fcan

ΔU 2 εWL 2 2 (H + z) (H + z + L sin α)

MATERIALS AND METHODS

Samples. Two samples, a multilayer graphene on silicon substrate and a polymer grating with heterogeneous surfaces, were prepared for subsequent experiments. The multilayer graphene was obtained by mechanical exfoliation and transferred to a silicon surface. The quality and thickness (number of layers) of the multilayer graphene sample was characterized by analyzing the intensity and shape of the Raman peaks, as shown in Figure 2. The D peak (caused by defects) is not detected, and the intensity of the G peak is higher than that of the 2D peak (which consists of 2D1 and 2D2 peaks). The Raman spectroscopy result reveals that the sample is composed of several graphene sheets without defects. As a protocol shown in Figure 3, the polymer grating is prepared by soft lithography and extrusion printing techniques. In the first step, a poly(dimethylsiloxane) (PDMS, Sylgard 184, A/B = 10:1) grating mold is fabricated by pouring PDMS into an AFM calibration grating

(3)

The total tip−sample capacitance can be represented as41 B


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its second flexural frequency with a tiny amplitude (the typical value is 0.5−1 nm) and periodically driven (far from its first flexural frequency) by the piezo-actuator to intermittently compress the sample with a maximum indentation force defined by the set point (Figure 4a-I). Then it is retracted from the sample until the maximum adhesion force point (Figure 4a-II) is reached, where the probe instantly separates from the sample surface. Finally, the probe is lifted up at a constant distance and maintained for a certain amount of time (usually twice the time constant of the probe, Figure 4a-III) to measure the CPD. The above protocol is repeated to measure subsequent data points. Figure 4b shows plots of the FD drive signal and probe responses during an oscillatory cycle. The drive signal is composed of a smallamplitude component tuned at the second flexural frequency of the probe and a large-amplitude component of a Gaussian signal tuned from several hundred to several kHz to drive the FD curve measurement. The data are acquired and then analyzed at high speed. The probe is freely vibrated at its second flexural frequency before snap-in point A and after point D, where the phase can be used to detect the electrostatic force for the CPD measurement. Although the phase signal is used as the feedback signal, the approach is sensitive to the electrostatic force. Because the electrostatic forces simultaneously affect the amplitude and phase of the feedback signal, they can be used as feedback signals in AM mode. When the probe separates from the sample surface (point C), the system will wait for a certain amount of time (which is greater than the time constant of the probe) before starting the KPFM measurement. It will prevent the system from being in a transition stage, thereby improving the measurement accuracy. The sample is indented from point A to pull-off point C, and the maximum contact force at point B is used for force feedback control. The nanomechanical properties of the sample are obtained by fitting the retract part of the FD curve from point B to point C. In the measurement of nanomechanical properties, the topographical errors and partial adhesion caused by electrostatic forces are eliminated by the presence of CPD compensation. Setup of the Multiparametric KPFM. As shown in Figure 5a, the MP-KPFM instrument mainly consists of a home-built AFM,45 an SPM dynamic control system with two independent oscillation controllers (OC4-I and OC4-II, Nanonis), control algorithms for FD spectroscopy, and a Kelvin controller. The cantilever is actuated by a superimposed signal: (i) composed of a Gaussian wave (UFD) with a frequency range from 0.5 to 2 kHz and a mechanical excitation signal (Um) at the second flexural eigenmode of the probe; (ii) an electrical excitation (UAC) fed between probe and sample at the second flexural eigenmode, which is generated from the mechanical excitation signal (Um) by a phase shifter with a phase difference of 90° between Um and UAC.42 The cantilever deflection signal is processed by the FD algorithm and the lock-in amplifier. The former controls the interaction force between the tip and sample, and the latter provides the phase shift to the Kelvin controller. Subsequently, the phase shift is minimized by regulating the offset voltage UDC that gives the magnitude of the UCPD.

Figure 2. Raman spectra of the scanned multilayer graphene. It was obtained with a Renishaw Raman spectrometer at 532 nm under ambient conditions. Inset: optical microscope image of a multilayer graphene (GR) sample, scale bar 10 μm.

Figure 3. Preparation of the polymer grating. (a) PDMS is coated onto an AFM calibration grating (step height 560 ± 2.6 nm, period 3.00 ± 0.01 μm) to produce a PDMS mold. (b) Peeling PDMS mold by adhesive tape. (c) PS is spin-coated onto the PDMS mold to emboss a PS grating. (d) Peeling PS grating by adhesive tape. (e) Printing LCA into the microchannels of the PS grating. (NT-MDT, TGS1-TGZ3), curing at 150 °C for 1 h in a vacuum drying oven, and peeling off of the calibration grating using adhesive tape. In the second step, polystyrene (PS, Aladdin, CAS 9003-53-6, P107085) is spin-coated onto the PDMS mold, held at 60 °C for 2 h in a vacuum drying oven, and peeled off of the PDMS mold using adhesive tape. In the last step, a light-curable adhesive (LCA, Loctite, 352) is printed into the PS grating channel using a micropipette and then cured by ultraviolet light (365 nm wavelength) after the LCA is sufficiently dispersed into the PS microchannels. Principle of Multiparametric KPFM. As shown in Figure 4a, the principle of MP-KPFM is described as follows: The probe is excited at

Figure 4. (a) Cartoon illustrating the principle of MP-KPFM: (I) maximum indentation force point, (II) pull off point (maximum adhesion force point), and (III) stable tip−sample distance. (b) Curves of the FD drive, tip−sample interaction force, and phase signal of the probe under the second resonance mode in an oscillatory cycle. Marked points B, C, and D correspond to positions I, II, and III, respectively. C


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Figure 5. (a) Schematic of the MP-KPFM instrument. A Gaussian wave (UFD) from an arbitrary waveform generator (AWG) is used for the FD drive. A mechanical excitation (Um) and an electrical excitation (UAC, which is 90° phase shifted Um) are applied between the sample and probe at the second flexural eigenmode. The FD algorithm is used to measure the mechanical properties of the sample, and the phase shift is used to measure the CPD between the tip and sample. The Kelvin controller minimizes the phase shift from the lock-in amplifier by regulating UDC to UCPD when the probe is lifted up. (b) Left top, displacement versus force plot; left bottom, displacement versus phase plot. Right graphs: encircled data of the right graphs is plotted as a function of the time. (c) The phase shift varies with the tip−sample bias voltages (UDC) at different mechanical excitations (Um) and a constant electrical excitation (UAC = 5 V). (d) Vibrational amplitude of the cantilever at the second flexural frequency versus the tip− sample bias with different tip−sample distances. The MP-KPFM instrument is first tested with a silicon probe (HQ: NSC18/Al-BS, MikroMasch) on multilayer graphene deposited on a gold 0coated silicon substrate (see section Samples) under ambient conditions. The second flexural frequency ( f 2) of the probe is 434.53 kHz (quality factor, Q = 343), and its stiffness is 2.87 N/m as calibrated by the Cleveland method.46 Drive signals Um and UAC have the same frequency of 434.53 kHz. The frequency of UFD is set as 1 kHz, and its amplitude is varied from 50 to 350 nm for the CPD measurement with different lift-up distances of the probe. Figure 5b shows a force/phase versus piezo-actuator displacement plot for fitting and further analysis. The force−displacement data are analyzed by the F-D algorithm. To obtain Young’s modulus, the retract force versus piezo-actuator displacement curve (from B to C) is converted to the force versus probe−sample distance curve and fitted with Derjaguin−Muller−Toporov (DMT) model,47 F − Fadh =

4 E* Rδ 3 3

The Gaussian drive signal is carefully constructed wherein the probe− sample separation time during the approach-and-retract cycle is more than twice the decay time of the second flexural mode (τ = 2Q/ω = 0.25 ms). In addition, to guarantee the accuracy of the CPD measurements, the mean value of the phase shift in the second-half part of the separation (part III) is used. Figure 5c is the phase versus tip−sample bias plot at different mechanical excitation strengths (lift-up distance DKelvin is set as 50 nm). The measurement is started without mechanical excitation (Um = 0) on the probe. In this case, the phase shift at UDC = UCPD becomes unstable and jumps by 180°. Fortunately, even with a small Um, the phase shifts monotonically with the varying tip−sample bias and is approximately linear in the vicinity of UDC = UCPD. With an increasing Um, the phase is less sensitive to the tip−sample bias. Thus, in our experiments, a compromised Um = 3 mV (its corresponding value of the mechanical amplitude is 0.8 nm) is used while considering the noise and the sensitivity. Figure 5d shows the amplitude versus tip−sample bias plot at different tip−sample distances when the cantilever is excited by only the electrostatic force (UAC = 5 V with a frequency of 434.53 kHz). With an increasing tip−sample distance (150, 200, and 300 nm and from 20 to 100 nm with an interval of 20 nm), the lowest point of the curve slightly moves toward the left as a result of the measured CPD decreases. This phenomenon is due to the increasing averaging effect (illustrated in Figure 1) and the higher work function of the goldcoated silicon substrate. In addition, it can be seen that the minimum value of the amplitude is never equal to zero. The nonzero amplitudes of probe are affected by many factors such as nonlinear intermittent

(7)

where F − Fadh is the force applied to the cantilever while taking into account the adhesion force, R is the tip apex radius, and δ is the indentation depth. The fitting results give the reduced elastic modulus E* whereas Young’s modulus of the sample can be calculated if Poisson’s ratio is known. The indentation depth (δ) is obtained by subtracting the deformation of the probe from the displacement of the piezo-actuator. From point D to point E, the probe is lifted up with a constant tip−sample distance, and the phase shift at the second flexural frequency due to the electrostatic force can be detected and compensated for by the Kelvin controller for the CPD measurement. D


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Figure 6. (a) CPD versus electrical excitation (UAC) and descriptive statistics of the original data (Um= 3 mV). (b) CPD between the probe and sample was measured when a varied voltage (from −4 to 4 V with a 100 mV step) was applied to the sample (Um = 3 mV, UAC = 5 V). The linear fit of the original data is y = 1.003x − 0.392. (b1) Statistical histogram and Gauss fit results of the measured step size. (b2) Schematic showing the measurement method. (c) The energy resolution of MP-KPFM was measured by applying a known voltage to the sample (Um = 3 mV, UAC = 5 V, and DKelvin = 25 nm). contact between the probe and sample, crosstalk of Um, and control errors.42,48 Then, the performance parameters of MP-KPFM under certain conditions are measured, which include the crosstalk of UAC, the measurement error of CPD, and the energy resolution. As shown in Figure 6a, the CPD was measured at the same point under different electrical excitation UAC, from 0.5 to 10 V with an interval of 0.5 V. The descriptive statistics of the original data is −391.04 ± 6.50 mV. The measurement of CPD is relatively stable when the UAC is different. In other words, the crosstalk of UAC is effectively compressed by introducing a phase difference of 90° between Um and UAC. Figure 6b shows the applied voltage (Ub) versus CPD wherein the Ub is known and varied in steps of 100 mV. The measured step size (acquired by subtracting from adjacent data) is −100.65 ± 3.41 mV. Therefore, the measurement error is −0.65 ± 3.41 mV. Figure 6c shows the CPD measured at several different step sizes of 25, 10, 5, and 2.5 mV. It is clear that the transition in the CPD is not obvious when the step size is 2.5 mV. However, for a step size of 5 mV, it has a clear and distinguishable transition. Its energy resolution is the same with traditional AM-KPFM.49

same area of the graphene−silicon surface. The corresponding height profiles (Figure 7c,h,m) give an almost equal height of the multilayer graphene. The magnitude of the multilayer graphene height is determined to be 12.18 ± 0.17 nm by averaging measurement results with the above three modes. Figure 7b,g,l shows the CPD map using the AM, FM, and MP modes, and Figure 7e,j,o shows the corresponding statistical histograms, respectively. CPD measurement results are given in Table 2. |UDC| measured with AM and MP-KPFM (both are sensitive to the electric force) is distinctly smaller than that measured with FM-KPFM due to the averaging effect; the tip is located on either graphene or silicon. As shown in Figure 7d,i,n, the averaging effect is obviously observed in the graphene−silicon junction, where the CPD profile transition is less steep in AM and MP-KPFM and the transition is sharper in FM-KPFM because of the weak averaging effect. Therefore, the graphene−silicon junction is more clearly imaged by FMKPFM. However, the CPD values are also different for AM and MP-KPFM. This is because the averaging effect varies at different tip−sample distances in AM mode, and the amplitude set point in tapping mode (AM and FM-KPFM) is 40 nm whereas the lift-up distance (DKelvin) is set at 50 nm in MPKPFM. Therefore, the measurement results of CPD are different. Figure 7p shows the map of the phase shift error, which is minimized (ideally zero) when regulating UDC to UCPD by the Kelvin controller. As shown in Figure 7r, the statistical histogram of the phase shift error gives a mean value of −0.08 ± 1.00°. The inverse sensitivity of the CPD measurement is calibrated to be 109.83°/V by fitting the linear part (−25−25°) of the phase shift versus the tip−sample bias curve (Figure 7s), which represents the response speed of the MPKPFM. Figure 7q illustrates that adhesion forces on the silicon and graphene surfaces are comparable except for the graphene− silicon junction. It is generated from the data of point C (marked in Figure 5b, the FD curve). Low adhesion forces are measured at the junction and are mainly attributed to the loose tip−sample contact on the junction area. The fitting results (with the double-peak Gaussian function) of the statistical histogram (Figure 7t) give mean adhesion forces of 21.50 ± 5.25 nN (solid line) and 25.28 ± 1.95 nN (dotted line) on the silicon−graphene junction and silicon/graphene surfaces, respectively.

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RESULTS AND DISCUSSION We performed two sets of experiments in this work. First, the sample of the multilayer graphene is imaged by conventional tapping mode (AM and FM) and MP-KPFM to demonstrate the performance of the developed method. Second, the polymer grating is scanned to validate the multiparametric imaging capability of the MP-KPFM instrument to concurrently map the topography, nanomechanical properties, and CPD in the single-pass fashion. Imaging the Multilayer Graphene with AM, FM, and MP Modes. To compare the results, the same region of the multilayer graphene on silicon substrate is scanned with AM, FM, and MP-KPFM. The amplitude set point in tapping mode (AM and FM) is 40 nm (which is 70% of the free amplitude). The amplitude and frequency of the UAC for AM and FM modes are set at 5 V and 434.53 kHz, 3 V and 6 kHz, respectively. Experimental parameters of the MP mode are described in Table 1. Topographical images in Figure 7a,f,k are respectively acquired using the AM, FM, and MP modes by scanning the Table 1. Experimental Parameters Used for the MP-KPFM

amplitude (lift-up distance) frequency

FD drive

Um

UAC

50 nm 1 kHz

3 mV 434.53 kHz

5V 434.53 kHz E


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Figure 7. Images of multilayer graphene on a silicon substrate by AM (a, b), FM (f, g), and MP (k, l, p, q) -KPFM. (a, f, k) Topographical images. The averaging height of the multilayer graphene is 12.18 ± 0.17 nm. (b, g, l) Surface potentials. (p, q) Maps of the phase shift error and adhesion force, respectively. (c, h, m) Height profiles along the lines are as indicated in a, f, and k, respectively. (d, i, n) Profiles of the surface potential along the lines as shown in b, g, and l, respectively. (e, j, o, r, t) Statistical histograms and Gauss function fitting results of images b, g, l, p, and q, respectively. (s) Linear fitting result of the phase versus tip−sample bias curve from −25° to 25° (Um = 3 mV). Scale bar: 1 μm. Image resolution: 272 × 272 pixels. (The R2 values for e, j, o, r, s, and t are 99.33, 99.62, 99.51, 99.33, 98.93, and 99.02%, respectively.)

Table 2. Measurement Results of CPD in Three Different Modes Si−Si Si−C

AM-KPFM

FM-KPFM

MP-KPFM

132.77 ± 40.84 mV −509.98 ± 46.69 mV

152.08 ± 29.79 mV −529.34 ± 32.20 mV

103.77 ± 41.52 mV −480.95 ± 38.54 mV

which is in accordance with the previously measured result.19 During scanning, appreciable deformation differences are generated on PS and LCA regions (Figure 8g), and indentation depths are measured to be 3.42 ± 0.42 and 15.34 ± 0.83 nm for PS and LCA (Figure 8j), respectively. The CPD map in Figure 8h also clearly represents the surface structure as a result of different surface potentials on PS and LCA regions. The mean values of the CPD for PS and LCA are measured to be 959.27 ± 108.68 and 417.05 ± 180.72 mV (Figure 8k), respectively. As shown in Figure 8l, the histogram of the phase shift error map (Figure 8i) gives a mean value of −0.05 ± 1.10°, which demonstrates that the CPD on the polymer grating surface is accurately mapped.

Imaging the Polymer Grating with MP-KPFM. To demonstrate the capability of MP-KPFM in simultaneously measuring CPD and nanomechanical properties in the singlepass fashion, a polymer grating with heterogeneous surfaces is tested. The topography in Figure 8a shows that the polymer is patterned with PS and LCA stripes. The corresponding height profile (Figure 8d) reveals that the LCA is about 60 nm below the PS with an arc-shaped transition at the junction due to the effect of surface tension during sample preparation. The polymer surface structure is clearly represented by the adhesion map (Figure 8b) with two obvious mean values of 27.80 ± 1.95 nN (PS) and 60.19 ± 1.52 nN (LCA), as distinguished in the adhesion force histogram in Figure 8e. The map of the reduced elastic modulus (elastic modulus*) of the polymer grating is shown in Figure 8c. The double-peak histogram (Figure 8f) illustrates that the PS has a elastic modulus* of 2.17 ± 0.31 GPa, whereas the LCA has an elastic modulus* of 0.41 ± 0.31 GPa. With a Poisson’s ratio of 0.33, Young’s modulus of PS is calculated to be 1.93 ± 0.28 GPa,

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CONCLUSIONS We have demonstrated a multiparametric KPFM, which combines FD-based AFM and AM-KPFM for simultaneous measurement of surface topography, CPD, and nanomechanical properties. First, multilayer graphene over a silicon substrate F


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Figure 8. Mapping of topographical and nanomechanical properties and CPD of a polymer grating by MP-KPFM. (a) Topography and (d) height profile (as indicated in a). (b, c, g, h, i) Adhesion force, elastic modulus*, deformation, surface potential, and phase shift error, respectively. (e, f, j, k, l) Corresponding statistical histograms and Gauss function fitting curves. Scale bar: 500 nm and image resolution: 256 × 256 pixels. (The R2 values of e, f, j, k, and l are 99.45, 99.32, 97.61, 99.53, and 98.63%, respectively.).

has been measured by AM, FM, and MP-KPFM. The results show that the CPD measured by AM and MP-KPFM are comparable; however, FM-KPFM shows higher CPD resolution because of its sensitivity to the force gradient. The scanning of the polymer grating shows that the developed method is versatile and robust because of its ability to measure the topography, CPD, and several mechanical properties such as the adhesion force and elastic modulus simultaneously. This method can provide fast and in situ multiparametric measurements such as the characterization of the electromechanical response of piezoelectric materials and the simultaneous mapping of the cell membrane potential and nanomechanical properties.

such as measurement errors; therefore, experimental data distributions can be presented by a Gaussian (or normal) function. Statistical histogram data is fitted by a unimodal or double-peak Gaussian function, wherein a unimodal Gaussian function is represented as

APPENDIX: STATISTICAL DATA PROCESSING Quantitative measurements of physical properties of the materials are often influenced by many independent factors

By the fitting, the expectation (a) and standard deviation (σ) of the Gaussian peaks are used to present experimental results as a ± σ.

f (x) = f (x0) +

A − (x − a)2 e σ 2π 2σ 2

(8)

A double-peak Gaussian function can be expressed as f (x) = f (x0) +

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A1 − (x − a1)2 A 2 − (x − a 2)2 + e e 2 σ1 2π σ2 2π 2σ1 2σ22 (9)

G


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(14) Dufrêne, Y. F.; Martínez-Martín, D.; Medalsy, I.; Alsteens, D.; Müller, D. J. Multiparametric Imaging of Biological Systems by ForceDistance Curve-Based AFM. Nat. Methods 2013, 10, 847−854. (15) Yue, T.; Park, K. H.; Reese, B. E.; Zhu, H.; Lyon, S.; Ma, J.; Mohler, P. J.; Zhang, M. Quantifying Drug-Induced Nanomechanics and Mechanical Effects to Single Cardiomyocytes for Optimal Drug Administration to Minimize Cardiotoxicity. Langmuir 2016, 32, 1909− 1919. (16) Xie, H.; Yin, M.; Rong, W.; Sun, L. In Situ Quantification of Living Cell Adhesion Forces: Single Cell Force Spectroscopy with a Nanotweezer. Langmuir 2014, 30, 2952−2959. (17) Galluzzi, M.; Biswas, C. S.; Wu, Y.; Wang, Q.; Du, B.; Stadler, F. J. Space-Resolved Quantitative Mechanical Measurements of Soft and Supersoft Materials by Atomic Force Microscopy. NPG Asia Mater. 2016, 8, e327. (18) Usov, I.; Nyström, G.; Adamcik, J.; Handschin, S.; Schütz, C.; Fall, A.; Bergström, L.; Mezzenga, R. Understanding Nanocellulose Chirality and Structure-Properties Relationship at the Single Fibril Level. Nat. Commun. 2015, 6, 7564. (19) Herruzo, E. T.; Perrino, A. P.; Garcia, R. Fast Nanomechanical Spectroscopy of Soft Matter. Nat. Commun. 2014, 5, 3126. (20) Dokukin, M. E.; Sokolov, I. Quantitative Mapping of the Elastic Modulus of Soft Materials with HarmoniX and PeakForce QNM AFM Modes. Langmuir 2012, 28, 16060−16071. (21) Wijeratne, S. S.; Penev, E. S.; Lu, W.; Li, J.; Duque, A. L.; Yakobson, B. I.; Tour, J. M.; Kiang, C.-H. Detecting the Biopolymer Behavior of Graphene Nanoribbons in Aqueous Solution. Sci. Rep. 2016, 6, 31174. (22) Dokukin, M.; Sokolov, I. High-Resolution High-Speed Dynamic Mechanical Spectroscopy of Cells and Other Soft Materials With the Help of Atomic Force Microscopy. Sci. Rep. 2015, 5, 12630. (23) Nonnenmacher, M.; O’Boyle, M. P.; Wickramasinghe, H. K. Kelvin Probe Force Microscopy. Appl. Phys. Lett. 1991, 58, 2921− 2923. (24) Kim, Y.; et al. Mechanical Control of Electroresistive Switching. Nano Lett. 2013, 13, 4068−4074. (25) Fuchs, F.; Caffy, F.; Demadrille, R.; Mélin, T.; Grévin, B. HighResolution Kelvin Probe Force Microscopy Imaging of Interface Dipoles and Photogenerated Charges in Organic Donor-Acceptor Photovoltaic Blends. ACS Nano 2016, 10, 739−746. (26) Honbo, K.; Ogata, S.; Kitagawa, T.; Okamoto, T.; Kobayashi, N.; Sugimoto, I.; Shima, S.; Fukunaga, A.; Takatoh, C.; Fukuma, T. Visualizing Nanoscale Distribution of Corrosion Cells by Open-Loop Electric Potential Microscopy. ACS Nano 2016, 10, 2575−2583. (27) Rohwerder, M.; Turcu, F. High-Resolution Kelvin Probe Microscopy in Corrosion Science: Scanning Kelvin Probe Force Microscopy (SKPFM) Versus Classical Scanning Kelvin Probe (SKP). Electrochim. Acta 2007, 53, 290−299. (28) Lee, H.; Lee, S. W.; Lee, G.; Lee, W.; Lee, J. H.; Hwang, K. S.; Yang, J.; Lee, S. W.; Yoon, D. S. Kelvin Probe Force Microscopy of DNA-Capped Nanoparticles for Single-Nucleotide Polymorphism Detection. Nanoscale 2016, 8, 13537−13544. (29) Moro, D.; Ulian, G.; Valdrè, G. Single Molecule Investigation of Glycine-Chlorite Interaction by Cross-Correlated Scanning Probe Microscopy and Quantum Mechanics Simulations. Langmuir 2015, 31, 4453−4463. (30) Salomão, F. C.; Lanzoni, E. M.; Costa, C. A.; Deneke, C.; Barros, E. B. Determination of High-Frequency Dielectric Constant and Surface Potential of Graphene Oxide and Influence of Humidity by Kelvin Probe Force Microscopy. Langmuir 2015, 31, 11339−11343. (31) Zilberberg, K.; Trost, S.; Schmidt, H.; Riedl, T. Solution Processed Vanadium Pentoxide as Charge Extraction Layer for Organic Solar Cells. Adv. Energy Mater. 2011, 1, 377−381. (32) Ellison, D. J.; Lee, B.; Podzorov, V.; Frisbie, C. D. Surface Potential Mapping of SAM-Functionalized Organic Semiconductors by Kelvin Probe Force Microscopy. Adv. Mater. 2011, 23, 502−507. (33) Liu, L.; Li, G. Electrical Characterization of Single-Walled Carbon Nanotubes in Organic Solar Cells by Kelvin Probe Force Microscopy. Appl. Phys. Lett. 2010, 96, 083302.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hui Xie: 0000-0003-4299-2776 Author Contributions

H.X. and H.Z. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported in part by the National Natural Science Foundation of China under grants 61573121 and 51521003 and the State Key Laboratory of Robotics and Systems (HIT) under grant SKLRS201619B.

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REFERENCES

(1) Zhang, Y.; Chen, Q.; Alivisatos, A. P.; Salmeron, M. Dynamic Charge Carrier Trapping in Quantum Dot Field Effect Transistors. Nano Lett. 2015, 15, 4657−4663. (2) Pingree, L. S. C.; Reid, O. G.; Ginger, D. S. Electrical Scanning Probe Microscopy on Active Organic Electronic Devices. Adv. Mater. 2009, 21, 19−28. (3) Guo, S.; Zhu, X.; Li, M.; Shi, L.; Ong, J. L. T.; Jańczewski, D.; Neoh, K. G. Parallel Control over Surface Charge and Wettability using Polyelectrolyte Architecture: Effect on Protein Adsorption and Cell Adhesion. ACS Appl. Mater. Interfaces 2016, 8, 30552−30563. (4) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, P.; Choi, J. Y.; Hong, B. H. Large-scale Pattern Growth of Graphene Films for Stretchable Transparent Electrodes. Nature 2009, 457, 706−710. (5) Wang, Y.; Yang, R.; Shi, Z.; Zhang, L.; Shi, D.; Wang, E.; Zhang, G. Super-Elastic Graphene Ripples for Flexible Strain Sensors. ACS Nano 2011, 5, 3645−3650. (6) Huang, M.; Pascal, T. A.; Kim, H.; Goddard, W. A.; Greer, J. R. Electronic-Mechanical Coupling in Graphene From in Situ Nanoindentation Experiments and Multiscale Atomistic Simulations. Nano Lett. 2011, 11, 1241−1246. (7) Savagatrup, S.; Makaram, A. S.; Burke, D. J.; Lipomi, D. J. Mechanical Properties of Conjugated Polymers and Polymer-Fullerene Composites as a Function of Molecular Structure. Adv. Funct. Mater. 2014, 24, 1169−1181. (8) Alsteens, D.; Newton, R.; Schubert, R.; Martinez-Martin, D.; Delguste, M.; Roska, B.; Müller, D. J. Nanomechanical Mapping of First Binding Steps of a Virus to Animal Cells. Nat. Nanotechnol. 2017, 12, 177−783. (9) Teo, B. K. K.; Wong, S. T.; Lim, C. K.; Kung, T. Y. S.; Yap, C. H.; Ramagopal, Y.; Romer, L. H.; Yim, E. K. F. Nanotopography Modulates Mechanotransduction of Stem Cells and Induces Differentiation Through Focal Adhesion Kinase. ACS Nano 2013, 7, 4785− 4798. (10) Strahl, H.; Hamoen, L. W. Membrane Potential is Important for Bacterial Cell Division. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 12281−12286. (11) Zhong, Q.; Inniss, D.; Kjoller, K.; Elings, V. Fractured Polymer/ Silica Fiber Surface Studied by Tapping Mode Atomic Force Microscopy. Surf. Sci. Lett. 1993, 290, L688−L692. (12) Radmacher, M.; Cleveland, J.; Fritz, M.; Hansma, H.; Hansma, P. Mapping Interaction Forces with the Atomic Force Microscope. Biophys. J. 1994, 66, 2159−2165. (13) Rosa-Zeiser, A.; Weilandt, E.; Hild, S.; Marti, O. The Simultaneous Measurement of Elastic, Electrostatic and Adhesive Properties by Scanning Force Microscopy: Pulsed-Force Mode Operation. Meas. Sci. Technol. 1997, 8, 1333−1338. H


Article

Langmuir (34) Palermo, V.; Palma, M.; Samori, P. Electronic Characterization of Organic Thin Films by Kelvin Probe Force Microscopy. Adv. Mater. 2006, 18, 145−164. (35) Cunningham, S.; Larkin, I. A.; Davis, J. H. Noncontact Scanning Probe Microscope Potentiometry of Surface Charge Patches: Origin and Interpretation of Time-Dependent Signals. Appl. Phys. Lett. 1998, 73, 123−125. (36) Kim, K. N.; Jung, Y. K.; Chun, J.; Ye, B. U.; Gu, M.; Seo, E.; Kim, S.; Kim, S.-W.; Kim, B.-S.; Baik, J. M. Surface Dipole Enhanced Instantaneous Charge Pair Generation in Triboelectric Nanogenerator. Nano Energy 2016, 26, 360−370. (37) Li, G.; Mao, B.; Lan, F.; Liu, L. Practical Aspects of Single-Pass Scan Kelvin Probe Force Microscopy. Rev. Sci. Instrum. 2012, 83, 113701. (38) Ziegler, D.; Stemmer, A. Force Gradient Sensitive Detection in Lift-Mode Kelvin Probe Force Microscopy. Nanotechnology 2011, 22, 075501. (39) Balke, N.; Maksymovych, P.; Jesse, S.; Kravchenko, I. I.; Li, Q.; Kalinin, S. V. Exploring Local Electrostatic Effects with Scanning Probe Microscopy: Implications for Piezoresponse Force Microscopy and Triboelectricity. ACS Nano 2014, 8, 10229−10236. (40) Li, Q.; Jesse, S.; Tselev, A.; Collins, L.; Yu, P.; Kravchenko, I.; Kalinin, S. V.; Balke, N. Probing Local Bias-Induced Transitions Using Photothermal Atomic Force Microscopy and Voltage Spectroscopy. ACS Nano 2015, 9, 1848−1857. (41) Li, C.; Minne, S.; Hu, Y.; Ji, M.; He, J.; Mittel, H.; Kelly, V.; Erina, N.; Guo, S.; Mueller, T. PeakForce Kelvin Probe Force Microscopy. Bruker Application Note #140 2013, 1−14. (42) Ziegler, D.; Rychen, J.; Naujoks, N.; Stemmer, A. Compensating Electrostatic Forces by Single-Scan Kelvin Probe Force Microscopy. Nanotechnology 2007, 18, 225505. (43) Zerweck, U.; Loppacher, C.; Otto, T.; Grafström, S.; Eng, L. M. Accuracy and Resolution Limits of Kelvin Probe Force Microscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 125424. (44) Wang, L.; Jin, K.; Gu, J.; Ma, C.; He, X.; Zhang, J.; Wang, C.; Feng, Y. A New Non-Destructive Readout by Using Photo-Recovered Surface Potential Contrast. Sci. Rep. 2014, 4, 6980. (45) Xie, H.; Hussain, D.; Yang, F.; Sun, L. Development of ThreeDimensional Atomic Force Microscope for Sidewall Structures Imaging with Controllable Scanning Density. IEEE/ASME T. Mech. 2016, 21, 316−328. (46) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. A Nondestructive Method for Determining the Spring Constant of Cantilevers for Scanning Force Microscopy. Rev. Sci. Instrum. 1993, 64, 403−405. (47) Adamcik, J.; Lara, C.; Usov, I.; Jeong, S.; Ruggeri, F. S.; Dietler, G.; Lashuel, H. A.; Hamley, W.; Mezzenga, R. Measurement of Intrinsic Properties of Amyloid Fibrils by the Peak Force QNM. Nanoscale 2012, 4, 4426−4429. (48) Kalinin, S. V.; Bonnell, D. A. Local Potential and Polarization Screening on Ferroelectric Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, 125411. (49) Melitz, W.; Shen, J.; Kummel, A. C.; Lee, S. Surface Science Reports Kelvin Probe Force Microscopy and Its Application. Surf. Sci. Rep. 2011, 66, 1−27.

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Multiparametric Kelvin Probe Force Microscopy for the Simultaneous

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