A Zinc Oxide Nanobeam Resonator for Ultrasensitivity Mass Detection

Nanomechanical resonators are expected to be exceptional sensors for high‐performance mass detection, mechanical sensing, and signal processing. In this paper, zinc oxide nanobeam resonators are produced based on single‐crystal ZnO nanowire, which has a typical diameter down to a few nanometers and the length of hundreds of micrometers. This resonator has the characteristics of high aspect ratio nanobeam structure and reliable material. It is observed that the resonance frequency of ZnO nanobeam resonator is up to 1.47 MHz with a high quality factor of 2300 at room temperature, which will play a key role in high‐sensitivity mass detection. The mass detection of ZnO nanobeam resonator is demonstrated by depositing platinum atoms on the middle of the beam, which shows a sensitivity of 11.13 Hz fg−1 indicating its ultrasensitive mass detection capability. In addition, according to the experiment, the molecular dynamics simulations for the resonator is established, which shows that the detection resolution down to 0.2 yg at room temperature can be realized based on this resonator. The results show that the ZnO nanobeam resonator has enormous potential in ultrasensitive detection for biosensing and gas sensing.


Introduction
In recent years, nanoelectromechanical systems (NEMS) have become prominent in science and technology novel innovation DOI: 10.1002/aelm.202300079 devices. [1][2][3] In particular, NEMS resonators have been proposed for ultrasensitive biosensing, [4][5][6] radiofrequency signal processing, [7][8][9] and as model systems for exploring quantum detections systems. [10,11] With the development of micro-nanofabrication technologies, nanoscale silicon resonators have been fabricated entirely using delicate top-down lithographic method. [12] However, this method is limited in materials choice, and more importantly, it is hard to drive the fabrication feature size smaller than 100 nm, which seriously hindered the sensitivity improvement. To address these challenges, low-dimensional materials produced by bottom-up methods have been taken advantage in NEMS resonators. Among many materials deemed suitable for NEMS resonators, 1D nanomaterials, such as silicon nanowires, [13,14] carbon nanotubes (CNT), [15][16][17][18] zinc oxide (ZnO) nanowires, [19][20][21][22] and silicon carbide (SiC) nanowires, [23][24][25] are promising candidates for realizing high-performance NEMS resonators because of its excellent mechanical properties; particularly high Young's modulus, small size and high volume ratio. Researchers have demonstrated suspended 1D nanomaterials as NEMS devices with potential applications as gas detection sensors, nano electronics, or quantum motion detectors. [26] Doubly clamped nanobeams have string-like properties with a simple bending mode shape function that renders them particularly suitable as ultrasensitive mass detector. [3,27] Moreover, the increase of sensitivity can be achieved by using nanobeam in resonator, Forsen et al. obtained 14.8 kHz resonance frequency shift through the absorption analytes of single glycerine drops. [28] Stachiv et al. provided analytical formulas for the frequency shift caused by an attached particle at an arbitrary location in the bridge and the cantilever configurations, which shows that the bridge can provide higher sensitivity than cantilever and the mass sensitivity can be further enhanced by detecting higher resonant frequencies. [29] In addition, the effects of the analyte properties, its size and the position of attachment on the accuracy of the determined analyte mass have been evaluated, which provides a significant physics insight behind the analyte adsorption indicating a great potential of the NEMS resonator in measurement of masses from single atom to chemical/biomolecule complexes. [30,31] Mass detection sensitivity of double clamped www.advancedsciencenews.com www.advelectronicmat.de nanotube or silicon nanobeams has been recognized using Cr atoms, naphthalene molecules, protein macromolecules and bacteriophage T5 capsid with mass spectrometry, which has demonstrated the great potential of NEMS resonator in high-sensitivity mass detection. [32][33][34][35][36][37] ZnO nanowires synthesized through chemical vapor deposition (CVD), showing extraordinary mechanical and electrical properties, which suggest it was suitable for highly sensitive NEMS resonators. [38] It has an ultralow mass density of 5.606 pg μm −3 and high Young modulus of 0.2 TPa, which are distinguished essential feature for high-sensitivity detection. Additionally, its excellent transfer properties such as a high transconductance of 1.9 mS, high electron mobility >1000 cm 2 V −1 s −1 and easy to eliminate the Schottky barrier, [39] makes them to be one of the best candidates as NEMS resonators, as they are allowed to transduce its vibration behaviors to electrical signals. Above all, NEMS resonator based on ZnO nanowires were expected to be high sensitivity and facilitate to integration with electronic circuits.
In this work, NEMS resonators, for ultrasensitive mass detection, were designed and demonstrated based on ZnO nanowires. Single-crystal ZnO nanowires synthesized based on the vapor solid mechanism by a CVD method with a diameter of a few nanometers and length of hundreds of micrometers were ideal materials for NEMS resonators. ZnO nanowires were suspended across microtrenches with predesigned electrodes by optical microscopy nanomanipulation technique and focus ion beam (FIB) technique. Benefitting from its outstanding electrical properties, the suspended ZnO nanowires was actuated to high frequency vibration by the interaction between the ZnO nanowires and the gate. At the same time, the resonance characterizations of ZnO nanobeam resonators were measured by the current flowing through ZnO nanobeam based on mixing principle. The resonance frequency of ZnO nanobeam resonators was measured to be 1.47 MHz with a quality factor of 2300 at room temperature, which contribute to the high-sensitivity mass detection combined with the ultralow size and mass of ZnO nanowires. The sensitivity of ZnO nanobeam resonators for mass detection was 11.13 Hz fg −1 , which was measured by depositing platinum (Pt) atoms on the middle of the beam. In addition, according to the experiment result molecular dynamics (MD) simulations was established for the resonator, which demonstrates that the detection resolution of ZnO nanobeam resonators was 0.2 yg at room temperature.

Field Effect Transistor Performance of ZnO Nanobeam
Herein, field effect transistor structure ZnO nanobeam resonators were designed and demonstrated based on the single crystal ZnO nanowires with high aspect ratio (length over diameter). The ZnO nanowire was manipulated under optical microscope to suspended across the microtrenches, and form a double clamped beam structure by depositing Pt atom at the interface between ZnO nanowires and electrodes, as displayed in Figure 1A,B. Particularly, top down lithographic methods were employed to fabricated microtrenches on quartz substrates, which would reduce the effect of parasitic capacitances (Note S1, Supporting Information). The metal-semiconductor-metal structure was built at both edge of the microtrench, which was built for improving the contact characterization between the nanowire and the electrode. The current-voltage (I sd −V sd ) curves with different gate voltage (V g ) of the ZnO nanobeam resonator was displayed in Figure 1C. The ZnO nanobeam resonator exhibited linear and symmetric I sd -V sd curves at different gate voltages, indicating ohmic contacts has formed between the ZnO nanowire and the electrode. In addition, as V g increased, I sd increased; and as V g decreased, I sd decreased. This result indicates that ZnO nanowires were typical n-type semiconductors. The I sd -V sd curves of ZnO nanowires also indicate that the devices operated in a depletion (normally ON) mode and transconductance g m was ≈500 nS for V g = 50 V.
The ZnO nanobeam resonator displayed the high field effect performance because of its crystal structure and phase purity of the bulk. So the crystal structure and phase purity of the bulk nanowire samples were assessed by X-ray diffraction. As displayed in Figure 1 D, all of the relatively sharp diffraction peaks are in good agreement with the standard ZnO wurtzite structure (a = 0.3250 nm and c = 0.5206 nm). The strongest peak (1 0 0) indicates that the growth direction of the ZnO nanowire was along the [0001] direction, in agreement with the below TEM images. Figure 1E shows the low-magnification TEM image of the ZnO nanowires and the corresponding selected area electron diffraction (SAED) pattern, which indicates the singlecrystal structure of ZnO nanowires and their [0001] growth direction. HRTEM examination ( Figure 1F) further indicates the single-crystal structure of the ZnO nanowire. All test results show that ZnO nanowires are purely wurtzite structure and do not have other impurities, which make it has high electron mobility, fast saturation electron drift rate, and suitable for manufacturing NEMS devices.

High Resonance Performance of ZnO Nanobeam Resonator
The resonance behavior of ZnO nanobeam resonators was investigated in a vacuum chamber with pressure below 10 −2 torr and room temperature (300 K). The suspended ZnO nanowires was accordantly actuated in high frequency vibration by electrostatic interactions between the nanobeam and the gate, which was induced from a combination voltage of AC voltage (V AC g ) and direct voltage (DC voltage: V DC g ) on the gate. The electrostatic interaction (F el ) between the nanobeam and the gate can be described as [33] where C g is the capacitance between the ZnO nanobeam and the gate and C ′ g = dC g ∕dz is the derivative of the gate capacitance with respect to the distance between the nanobeam and the gate. The first part of Equation (1), 1 2 is the electrostatic force induced by the DC voltage applied on the gate, which can be employed to tune the resonance frequency by stretching the nanobeam. The second part of Equation (1), C ′ g V DC g V AC g , is an alternating electrostatic force, which actuates the nanobeam into high frequency vibration. The last part of the right side of Equation (1), induced from the AC voltage applied on the gate which is much smaller than DC voltage. It is so small that it can be neglected (detail information was provided in Note S2 in the Supporting Information). Benefitting from its outstanding transfer property, the vibration behaviors of the nanobeam were investigated with the current flowing through the nanobeam based on the mixing principle, as shown in Figure 2A. The mixing current (I mix ) was depicted as where dG/dV g is the transconductance of the nanobeam and C g is the capacitance modulation induced by the mechanical vibration of ZnO nanobeams. The first part of Equation (2) is the component induced from the AC voltage applied on the gate, which exists all the time due to the capacitance coupling between the ZnO nanobeam and the gate. The second part of Equation (2) is the component induced from the capacitance modulation generated from the displacement of the ZnO nanobeam, which only exist as the nanobeam was in motion. Thus, there will be a distinguish current feature as the vibration frequency approaches the eigen frequency of the ZnO nanobeam, which was induced by the nonlinear displacement at resonance frequency.
As an AC voltage of 500 mV applied on the source and a combination of 500 mV AC voltage and 10 V DC voltage applied on the gate, a clear peak in comparison with a slowly changing background in the current-frequency curve was observed, as shown in Figure 2B, corresponding to the beam resonance mode of the doubly clamped ZnO nanobeam. This feature was attributed to the resonance motion of the ZnO nanobeam, which modulated the capacitance between the ZnO nanobeam and the gate; whereas the background current was due to modulation of the gate voltage. The response fits well to a Lorentzian function with a normalized linewidth Q −1 = Δ / = 1/915, a resonant frequency 0 =1.528 MHz. The resonance frequency of ZnO nanobeam resonators can be depicted as [12] where k is the spring constant and m eff = 0.735 r 2 l is the effective mass of ZnO nanobeams. The double clamped ZnO nanobeam resonator can have large frequency tuning range owing to its high aspect radio, which was facilitate to control the spring constant by applying gate voltage, strain, heat, etc. In this work, the resonance frequency of ZnO nanobeam resonators was tuned by DC voltage applied on the gate. Figure 2C shows the response of the resonance frequency f 0 to the gate voltage V DC g for ZnO nanobeam resonators. The resonance frequency increase as the magnitude of V DC g increases, which was attribute to the elastic hardening of the spring constant. While DC voltage applied on the gate increases, the ZnO nanobeam would be pulled more to the gate thus induces tension inside the ZnO nanobeam increasing. The correlation between the DC voltage applied on the gate and the resonance frequency can be expressed as where E Y is the Young modulus, I is the moment of inertia about the longitudinal axis of the ZnO nanobeam, L is the length, S is the cross sectional area of the ZnO nanobeam, is the mass density of the ZnO nanobeam, T 0 is the initial built-in tension of the ZnO nanobeam induced from fabrication process, z e is the static deflection at the center of the ZnO nanobeam, C ′ g is the second derivative of the capacitance.
According to Equation (4), the z 2 e term dominates the spring constant hardening owing to the stretching of ZnO nanobeams, www.advancedsciencenews.com www.advelectronicmat.de where the ZnO nanobeam fabrication residual strain was slight. [40] The resonance frequency of device A and device B (Morphology of Device A and device B was provided in Note S3 in the Supporting Information) were tuned from 1.2 to 1.57 MHz and 0.87 to 1.11 MHz by controlling gate voltages, thus the frequency tuning range is larger than 25% for device A and device B. The resonance frequency of device C (Morphology of Device C was provided in Note S3 in the Supporting Information) was tuned from 1.45 to 1.53 MHz by controlling gate voltages, and the frequency tuning range is about 5.5%. The monotonically increasing frequency as gate voltage rises indicates that the residual stress ZnO nanobeam resonator was minimal, which makes it appropriate for high-sensitivity detection. [40][41][42][43][44][45][46] The frequency tuning range of device A and device B is larger than that of device C, which was caused by the difference distance between the ZnO nanobeam and the gate. The distance between ZnO nanobeams and the gate of device A and device B is about 1.25 μm, and that of device C is about 5.12 μm. Since the gate voltage induced a larger center point deflection z e as the distance between the ZnO nanobeam and the gate is smaller, which result in a larger frequency tuning range due to the spring constant hardening. At the same time, the increased larger center point deflection z e would lead to an increasing energy dissipation during the vibration cycle of the ZnO nanobeams, which result in the decrease of the quality factor of the ZnO nanobeam resonator. However, the quality factor of device C was still up to about 2300 at room temperature when V DC g was 15 V, which enhances the prospects of the ZnO nanobeam for applications in the high-sensitivity detection.
An important parameter of NEMS resonators is the quality factor Q, which was adopted to characterize the ratio of the energy stored in the resonator to the energy lost per cycle induced from damping. Maximizing Q is always dedicated for device applications, which would lead to better frequency selectivity and a smaller power required to sustain the oscillation arisen from a sharper resonance peak and slower energy dissipation. Because air damping is one of the most important energy dissipation, the dependence of the resonator characterization on the pressure was investigated in the vacuum chamber. Figure 2E displays the current frequency curve with pressure from 0.2 to 0.7 torr and Figure 2F summarizes the relationship between the Q factor and the pressure. Q factor decreases with pressure increasing, and the current peak still distinct from the background current when pressure was above 0.7 torr. These experiments suggest that Q factor was high as the pressure was low and air damping had a strong influence on Q factor, which was in consistent with calculations. [47]

High-Sensitivity Mass Detection Performance
ZnO nanobeam resonators exhibit excellent properties, such as small size, high volume surface, and high resonant frequency, which renders them promising for a wide range of applications. The application of ZnO nanobeam resonators in the field of mass detection was investigated by calculations, simulations, and experiments. The resolution of the ZnO nanobeam resonator mass sensor is where m, m eff , f 0 , and f 0 represent the minimum detectable mass, effective mass of the ZnO beam, resonance frequency, and frequency deviation, respectively. The small size and excellent resonance properties of the doubly clamped ZnO nanobeam resonators enable them to have substantial potential in the field of mass detection. The high mass detection sensitivity of ZnO nanobeam resonators was demonstrated by FIB technique. Pt atoms were deposited onto the middle of the ZnO nanobeam by FIB technique because one can control the mass and position of the Pt atoms deposited onto the nanobeam with delicately FIB technique, as shown in Figure 3A,B (more details in Note S4 in the Supporting Information). One can obtain the mass of the Pt atoms deposited onto the middle of the ZnO nanobeams by the deposition parameters of the FIB and the morphology of the Pt atoms deposited onto the middle of the ZnO nanobeam. The mass of ZnO deposited on the ZnO nanobeam is 1.716 pg each time, calculated as follows: × l × w × t ( is the density of Pt: 21.45 g cm −3 , l is the length of the deposited mass: 2 μm, w is the width of the deposited mass: 200 nm, and t is the height of the deposited mass: 200 nm). Figure 3C shows the typical current-frequency curve of the ZnO nanobeam resonator, as 1.716 pg Pt atoms were deposited onto the middle of the ZnO nanobeam each time, the resonance frequency shifts as well (more details in Note S4 in the Supporting Information). Figure 3D shows the shift of the resonant frequency as a function of the mass of deposited Pt atoms. The slope gives the mass responsivity, ℜ= 11.13 Hz fg −1 , which indicates the ZnO nanobeam resonator has excellent potential in the field of mass detection.

ZnO Nanobeam Resonator for Atomic Scale Mass Detection
The zinc nanobeam resonator exhibits excellent potential in mass detection and might be used in atomic scale mass detection. To demonstrate the ZnO nanobeam NEMS resonator capacity for atom detection, the vibrational characterization of ZnO nanobeam resonators were investigated by MD methods based on the Forcite module in Materials Studio. Figure 4A shows molecular dynamics model of the ZnO nanobeam resonator, and Figure 4B shows the ZnO nanobeam resonator molecular dynamics model with an attached Pt atom. The clamped ends are marked in yellow block; the zine oxide nanobeam had a length L (beam length defined as the distance between two clamped ends) of 9.8 nm and a diameter of 1.0 nm.
The vibrational behavior of the ZnO nanobeam was performed by using the COMPASS force field (Condensed-Phase Optimized Molecular Potentials for Atomistic Simulation Studies) in the Forcite module at room temperature (298 K). The Forcite Module is a tool that one can use to perform a wide range of molecular dynamics calculations (such as single-point energy calculations, geometry optimization, and molecular dynamics) using a classical force field based on simulation techniques. First, geometry optimization was taken advantage to minimize the energy stored in the ZnO nanobeam resonator structure by adjusting the relative distances between a group of atoms. The cell parameters and atomic coordinates were adjusted by running the iteration process, which was a cascade of methods that uses successively steepest descent and conjugate gradient algorithms. The convergence quality was set as ultrafine; in which energy was 2 × 10 −5 kcal mol −1 , the maximum force was 0.001 kcal mol −1 per Å, and the maximum displacement was 1 × 10 −5 Å. The cell parameters and atomic coordinates were determined until the minimization of the total energy of the structure. Second, displacement excitation was used to drive the ZnO nanobeam into high frequency vibration. A silicon nanotip was positioned under the ZnO nanobeam, and its geometric center was located in the xy center of the nanobeam. In the beginning, the tip was nearly approaching the nanobeam vertically; thus, Van der Waals force was applied to the middle of the nanobeam. After geometry optimization, a deflection of ≈10% of the ZnO nanobeam diameter was generated. The 10% deflection actuates the ZnO nanobeam into high frequency vibration mode without causing changes in the values of f 0 and Q (vibration movies displayed by Movies S1-S5 in the Supporting Information). During this period, the system was held under constant temperature until the potential energy saturated to a constant value. At the final stage of the simulation procedure, the tip was removed and executed a dynamics task to analyze the ZnO nanobeam vibration characterizations. The ZnO nanobeam was actuated to vibrate under an NVE ensemble without disturbance (microcanonical ensemble) for 50 PS with a time step of 1 fs, and the convergence quality was set as ultrafine. The kinetic energy was recorded to describe the vibrational characteristics of the ZnO nanobeam, which absorbed Pt atoms (0.324 yg per atom), as displayed in Figure 4C. Thirdly, a fast Fouriertransform (FFT) technique was used to extract the resonance frequency of the ZnO nanobeam from the periodicity kinetic energy curve, as shown in Figure 4D. Finally, the least square algorithm of fitting was adopted to calculate the mass detection sensitivity of the ZnO nanobeam resonator. Figure 4E indicates that 2.48 GHz would shift upon per yg Pt atom adsorbed on the beam. The mass resolution was the distinct resonance frequency shift resulted from the amount of mass adsorbed on the beam. In this study, 0.5 GHz was chosen as resonance frequency shift standard to evaluate the mass detection resolution of ZnO nanobeam resonators for comparative and qualitative analysis purposes. Thus, based on the standard chosen, the mass detection resolution of ZnO nanobeam resonators was denoted to be about 0.2 yg.

Conclusions
In conclusion, an ultrasensitive ZnO nanobeam resonator with low residual strain was designed and demonstrated based on single-crystal ZnO nanowires, which possesses outstanding mechanical and electrical properties with a diameter of tens of nanometers and length of hundreds of micrometers. The resonance behavior of ZnO nanobeam resonators was investigated in a vacuum chamber with pressure from 10 −3 to 10 −1 torr. The resonance frequency of ZnO nanobeam resonator was observed to be 1.47 MHz with a Q factor up to 2300 at room temperature, whose frequency tuning range is larger than 25% by control DC voltage applied on the gate. Owing to the remarkable resonance characterization, ZnO nanobeam resonators were expected to be an ultrasensitivity mass detection sensor. The sensitivity of ZnO nanobeam resonator was evaluated to 11.13 Hz fg −1 by depositing Pt atoms onto the nanobeam with FIB technique. In addition, in accordance with the experiment result the mass detection capacity of ZnO nanobeam resonators was studied by MD simulation, which demonstrates that the resolution of ZnO nanobeam resonators was 0.2 yg. These results indicate that ZnO nanobeam resonators were capable to serve as an ultrasensitivity mass detection sensor. Furthermore, combined with the large surface to volume ratio and absorbing properties of ZnO nanowires, ZnO nanobeam resonators exhibit great potential in ultrasensitivity biosensor and gas sensor.

Experimental Section
Fabrication of ZnO Nanobeam Resonators: Single-crystal ZnO nanowires with diameters varying from 1 nm to 2 μm were synthesized via the vapor solid mechanism in a CVD method. As displayed in Figure 5, optical microscope nanomanipulation and FIB technique were adopted to prepare the ZnO nanobeam resonator, where the axial internal stress induced from residual strain was fully considered during the fabrication processing. The synthesized ZnO nanowires were ultrasonically dispersed in ethanol for several minutes to separate and disperse the individual ZnO nanowire from the ZnO nanowires cluster (as displayed in Figure S7A,B in the Supporting Information). A drop of the solvent containing ZnO nanowire was dispersed onto the grid, and then a tungsten needle was used to pick up a single ZnO nanowire and place it on a resonator device quartz substrate with predesigned source, drain, and gate electrodes (as displayed in Figure S7C,D in the Supporting Information), where the ZnO nanowire was in free state and the residual strain was released by overcoming the adhesion force between the nanowire and the electrode. To clamp ZnO nanowires suspended across microtrenches, an FIB technique (16 keV Ga + with a 25 pA current aperture) was carried out to deposit Pt onto the interface between the nanowire and electrodes. To fixed the ZnO nanowire smoothly, two rectangle Pt blocks were deposited at both sides of the ZnO nanowire on electrodes, then Pt atoms were deposited on the interface between the ZnO nanowire and the Pt blocks, which was displayed in Figure S7E (Supporting Information). At last, as displayed in Figure S7F (Supporting Information), the lateral ZnO nanowire was cut off to improve the resonance performance of the ZnO nanobeam resonator.
Characterizations of ZnO Nanobeam Resonators: Semiconductor characterization instrument and probe station were adopted to analyze the transfer property of the ZnO nanobeam resonator. X-ray powder diffractometer and transmission electron microscope (TEM) were employed to analyze the crystal structure of the ZnO nanowire. Based on mixing principle, the resonant characterization of the ZnO nanobeam resonator was evaluated with the help of lock-in amplifier. The mass detection sensitivity of ZnO nanobeam resonators was investigated by depositing Pt atoms on the middle of the beam. In addition, the atom scale mass detection capacity of the ZnO nanobeam resonator was demonstrated with molecular dynamics simulation by Materials Studio 2019.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.