Low-noise humidity controller for imaging water mediated processes in atomic force microscopy I. Gaponenko, L. Gamperle, K. Herberg, S. C. Muller, and P. Paruch Citation: Review of Scientific Instruments 87, 063709 (2016); View online: https://doi.org/10.1063/1.4954285 View Table of Contents: http://aip.scitation.org/toc/rsi/87/6 Published by the American Institute of Physics Articles you may be interested in Towards reversible control of domain wall conduction in Pb(Zr0.2Ti0.8)O3 thin films Applied Physics Letters 106, 162902 (2015); 10.1063/1.4918762 Ferroelectric domains in epitaxial PbxSr1-xTiO3 thin films investigated using X-ray diffraction and piezoresponse force microscopy APL Materials 4, 086105 (2016); 10.1063/1.4960621 A virtual instrument to standardise the calibration of atomic force microscope cantilevers Review of Scientific Instruments 87, 093711 (2016); 10.1063/1.4962866 G-mode magnetic force microscopy: Separating magnetic and electrostatic interactions using big data analytics Applied Physics Letters 108, 193103 (2016); 10.1063/1.4948601 Interface modulated currents in periodically proton exchanged Mg doped lithium niobate Journal of Applied Physics 119, 114103 (2016); 10.1063/1.4943934 Calibration of higher eigenmodes of cantilevers Review of Scientific Instruments 87, 073705 (2016); 10.1063/1.4955122 REVIEW OF SCIENTIFIC INSTRUMENTS 87, 063709 (2016) Low-noise humidity controller for imaging water mediated processes in atomic force microscopy I. Gaponenko,a) L. Gamperle, K. Herberg,b) S. C. Muller, and P. Paruch DQMP, University of Geneva, 24 Quai E. Ansermet, 1211 Geneva 4, Switzerland (Received 25 April 2016; accepted 7 June 2016; published online 21 June 2016) We demonstrate the construction of a novel low-noise continuous flow humidity controller and its integration with a commercial variable-temperature atomic force microscope fluid cell, allowing precise control of humidity and temperature at the sample during nanoscale measurements. Based on wet and dry gas mixing, the design allows a high mechanical stability to be achieved by means of an ultrasonic atomiser for the generation of water-saturated gas, improving upon previous bubbler-based architectures. Water content in the flow is measured both at the inflow and outflow of the fluid cell, enabling the monitoring of water condensation and icing, and allowing controlled variation of the sample temperature independently of the humidity. To benchmark the performance of the controller, the results of detailed noise studies and time-based imaging of the formation of ice layers on highly oriented pyrolytic graphite are shown. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4954285] I. INTRODUCTION In the field of materials science, the control of environmental conditions—such as humidity and temperature— is essential for optimising the quality of measurements, and thus for an improved understanding of the underlying physical processes. The presence of water in air and its adsorption on surfaces are a well known phenomenon, studied as much for its fundamental interest1,2 as for its technological applications,3 where its effects on the adhesion, friction, and wear of solid interfaces play a particularly significant role in nanotribology. On silicon dioxide, a material of high industrial importance, the evolution of the adsorbed water layer has been mapped out in detail as a function of humidity,4,5 demonstrating a complex behaviour with initial ice-like structure, followed by the appearance of liquid-like water, leading to a non-trivial growth of the layer thickness. Such studies have been determinant for the development of new devices, such as sensors exploiting the complex water interactions on their surfaces.6 However, high precision humidity control is also crucial for studies in other fields, such as at multiple observation scales in biology, where the properties of organic samples can be greatly affected by the presence of surface water or more broadly ambient humidity. At the macroscopic scale of living organisms, for example, the attachment ability of certain spiders is strongly humidity dependent,7 while at the microscopic scale bacterial cell morphological properties likewise change with humidity,8 and even at the nanoscale significant effects of surface water have been shown in DNA—which at high humidities regains mobility and exhibits conformational changes.9 The technique of choice for measurements of the nanoscale properties of materials and their interaction with surface adsorbates is atomic force microscopy (AFM). By means of a)Electronic mail: email@example.com b)Present address: LIFMET, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland. 0034-6748/2016/87(6)/063709/6/$30.00 a nanometrically sharp scanning probe tip, interactions such as adhesion and friction have been studied as a function of environmental conditions,3,10 showing distinct changes with increasing ambient water content. In order to perform such studies, the sample and scanning probe are usually isolated in humidity-controlled cells, with constant relative humidity (RH) mediated either by saturated saline solutions11,12 or by a mixing flow of wet and dry gas.13–18 In the former case, only a limited number of specific humidities is available due to the dynamic evaporation of the saturated solution—requiring changes in sample temperature to accommodate the missing values. In the latter, although the humidity can be changed continuously from essentially 0% RH to full saturation, the usual bubbler design severely limits measurements of local properties at the nanometer scale due to the mechanical noise induced. In this paper we describe a low-noise humidity control apparatus tailored for scanning probe microscopy applications, where stability and low noise are essential. In order to achieve this, whilst keeping the capability to vary the humidity at the sample without changing its temperature or stopping the data acquisition, we have developed a continuous flow mixing humidifier using a water atomiser as the source of wet gas. As a benchmark of the high performance of this design, we show the noise characteristics of the humidifier integrated with a commercial Asylum Research Cypher ES system and demonstrate high resolution time-based imaging of ice growth on highly oriented pyrolytic graphite (HOPG). II. HUMIDITY CONTROLLER A. General design The humidity controller design, shown in Fig. 1(a), is based on a continuous flow mixing mechanism. The basic idea behind it is the mixing of a dry and a wet gas flow, the ratio of which is precisely determined for a target value of 87, 063709-1 Published by AIP Publishing. 063709-2 Gaponenko et al. FIG. 1. (a) Working principle of the humidity controller. A dry N2 flow is split into two by means of a T-junction and injected into REG_DRY and REG_WET regulators coupled to computer-controlled motors. The REG_WET flow is saturated with H2O by means of an ultrasonic atomiser and then recombined with the REG_DRY flow by means of a T-junction. The combined flow then passes through a mixing chamber, MIX_INJ, where the humidity and temperature are measured, and is then injected into the AFM fluid cell. Finally, the output from the cell passes through a second mixing chamber, MIX_EXH, for its humidity and temperature to be measured again before it is exhausted out of the system. (b) Inside view of the humidity controller showing the pneumatic cabling and the monitoring electronics. humidity. In our system, dry N2 gas at a pressure of 4 bar (≈60 psi) enters the apparatus and is split into two pathways by means of a T-junction. The two resulting flows are regulated by Porter VCD 1000 variable constant flow differential controllers (REG_DRY and REG_WET) with interchangeable flow elements—110 cc/min in our case, as we do not wish to have too high a flow in the AFM—coupled to Trinamic PD-1140 stepper motors for a precise computer control of the mixing ratio. The REG_WET flow is water-saturated by passing through the atomiser mechanism shown in Fig. 2 and discussed below, and is then recombined with the REG_DRY flow by means of the second T-junction. The recombined flow is then injected into a passive mixing chamber (MIX_INJ) designed to break the non-mixing laminar flows before entering the AFM fluid cell. From here, the flow then travels back into the humidity controller and passes through a second passive mixing chamber (MIX_EXH) before exiting through the exhaust. In both mixing chambers the temperature and relative humidity of the flow are measured by means of Sensirion SHT1519 sensors coupled to an Arduino Leonardo board for data display and computer acquisition. Because of the dual-sensor continuous flow design, the controller can not only be easily adapted onto virtually any enclosed volume, but the humidity can also be varied programmatically without the need for a human interface. Rev. Sci. Instrum. 87, 063709 (2016) FIG. 2. (a) Schematic diagram of the atomiser unit, describing both the dry/wet N2 exchange chamber and the electric wiring. The ultrasonic element power is controlled by means of the POWER_POT potentiometer and can be switched on/off with the ATOM_SW switch. Active airflow cooling composed of a heat-sink and two fans—FAN_IN and FAN_OUT—can be switched on with the FAN_SW switch. The external pulse width modulation board can be plugged into the system with a 3.5 in. connector on PWM_CONN and controls the PWM_RLY relay to minimise heating due to the thermal dissipation of the piezoelectric bimorph. (b) Once turned on, the ultrasonic element saturates the chamber with a dense, visible water mist. (c) Outside view of the atomiser unit. The red pneumatic tubing brings the dry N2 in and carries the H2O-saturated N2 out of the system. Moreover, with the delocalised sensors, the temperature of the probed sample can be varied without adverse effects on the measurement and control of the humidity. This modus operandi can therefore give additional insight into the processes happening in the system under measurement— mechanisms such as condensation and ice formation can be investigated at a constant flow rate and water content of the incoming gas, which can be calculated from the humidity measured at MIX_INJ, with changes to the water content monitored by a drop in the exhaust humidity at MIX_EXH. B. Atomiser For a mixing type humidifier, a steady supply of gas fully saturated in H2O is necessary. In previous designs, this requirement was usually fulfilled by the use of a water bubbler, in which dry gas is saturated as it passes through liquid H2O as bubbles.12–18 We initially tested such a system using Supelco 64712-U/64834-U threaded midget bubblers as the source of H2O-saturated gas.20 However, our tests showed that although the required humidities were reachable, the bubbler was found to induce unacceptable levels of mechanical noise at the cantilever, resulting in periodic nanometer-scale jumps in the topography as shown in Fig. 4(e) and discussed below. To counteract this problem, a wet sponge filter was also tested20 but due to its obstructions of the gas flow it could not 063709-3 Gaponenko et al. provide for fast enough stabilisation, although it is a viable cost-effective alternative for non-time-critical measurements where a fast response is not required. Thus, an atomiser design was favoured, based on the 8 TDK NB-80E-01-H atomiser unit. In this configuration, a piezoelectric bimorph is excited at ultrasonic frequencies (2.35-2.6 MHz), generating a stream of microdroplets above the water surface, fully saturating the surrounding gas with H2O. The design is shown in Fig. 2(a), with the ultrasonic atomiser unit represented in orange. The latter produces a dense water mist, as shown in Fig. 2(b), which saturates the input dry N2 flux before it leaves the chamber, as schematically shown by the blue exit tube. A splash guard is present in order to avoid drops of water blocking the tubing. The density of the water mist is controlled by means of a potentiometer (POWER_POT), and the generation of mist can be switched on or off mechanically (ATOM_SW). In order to avoid the heating of the water in the chamber due to the thermal dissipation of the piezoelectric bimorph, two countermeasures were implemented. First, an optional active cooling system composed of a heat sink and fans (FAN_IN and FAN_OUT) can be switched on (FAN_SW). This provides partial thermal dissipation in conjunction with a passive aluminium heat sink increasing the thermal exchange between the atomiser unit and the airflow. Second, because of the relatively low N2 flux and therefore small volume that needs to be maintained at saturation, continuous atomising is unnecessary and even counterproductive due to heat dissipation. Thus, a simple pulse width modulation controller was implemented with an Arduino Leonardo board in order to modulate the on/off state of the atomisation by means of a relay (PWM_RLY) to achieve the desired output. C. Integration and characterisation For tests as well as actual measurements, the controller was integrated into a commercial Peltier cooler/heater fluid cell mounted on an Asylum Research Cypher ES atomic force microscope. This setup is capable of delivering temperatures between −8 ◦C and +120 ◦C (with a software controlled closed loop feedback allowing for a reliable and stable sample temperature between 0 ◦C and 100 ◦C). By virtue of this setup, we could harness the capability of independent control over both temperature and humidity at the sample. First, noise measurements were performed in order to characterise the effect of the humidity controller on the mechanical stability of the atomic force microscope. A thin film of monocrystalline epitaxial PbTiO321 was imaged under four different conditions: no flow, dry gas flow, atomiser wet flow, and bubbler wet flow.22 The results, shown in Figs. 3(a)-3(d), demonstrate the mechanical stability of the atomiser compared to the classical bubbler design. Images taken with the atomiser in operation (Fig. 3(c)) show identical topography to those performed with no flow on dry gas, with a subnanometer resolution allowing unit cell high terraces (∼4 Å high and atomically flat) in the film to be clearly distinguished. In contrast, as can be seen in Fig. 3(d), the pressure waves generated by the bubbler can be seen as periodic ≈5 nm jumps in the measured topography. To confirm this observation, the Rev. Sci. Instrum. 87, 063709 (2016) FIG. 3. Topography of an epitaxial PbTiO3 thin film as imaged by low amplitude AM-AFM during (a) no flow, (b) dry gas flow, (c) atomiser wet flow, and (d) bubbler wet flow. As can be seen, the bubbler induces periodic ≈5 nm jumps in the topography. (e) The spectral response of the vertical deflection of the fully retracted AFM tip, measured by laser reflection onto a quadrant photodetector, demonstrates the periodic nature of the mechanical noise due to bubbler-induced pressure waves, which is absent in identical measurements carried out on the closed fluid cell with no flow, during dry gas flow, as well as a wet flow generated by the atomiser. vertical deflection signal of the AFM photodetector was then acquired as a function of time, with the tip retracted 50 µm from the surface. The resulting spectral response is shown in Fig. 3(e). As highlighted on the spectrum (red), the bubbler indeed generates a periodic noise depending on the frequency at which bubbles are expelled from the nozzle, whereas no visible difference can be seen between the spectra produced when the measurement is performed with the atomiser, with dry flow, or with no flow through the fluid cell. D. Control and measurement The measurement of humidity in the injection chamber MIX_INJ and the exhaust chamber MIX_EXH is performed by Sensirion SHT15 sensors interfaced to an Arduino Leonardo board. The sensors are polled once per second, and the resulting humidities and temperatures are displayed on the onboard LCD screen for manual operation. Ideally, the Arduino is connected to a computer and outputs the data acquired from the sensors in addition to its other functions. Thus, the humidities and temperatures can be saved, plotted, and used for active closed loop control. 063709-4 Gaponenko et al. FIG. 4. Relative humidity and temperature measured in the injection and exhaust mixing chambers as a function of time during a six day experiment. As can be seen from the time axis, the humidities were cycled five times up to ∼85% RH during the measurement. The small changes in temperature reflect the variations in ambient conditions in the laboratory. The increase in the baseline humidity as a function of time can be explained by the incomplete desorption of water in the tubing. Finally, the difference in levels between the humidities measured in the injection and exhaust chambers is due to the significant system load, composed of not only the fluid cell but also over two meters of tubing. The control of the humidity itself is done by adjusting the two high precision 14-turn flow regulators REG_DRY and REG_WET, allowing for a good finesse in the control of dry and wet gas flow between 0 and 110 ml/min each—a range which can be changed by replacing the flow elements in the regulators to adjust for the system size and required finesse.23 Although they can be adjusted manually, one of the main advantages of the controller design is that the two motors are coupled to the regulators in order to allow for automatic control. The motor control is performed by a set of LabVIEW scripts, which range from simple open-loop control to closed-loop regulation of humidity, depending on Rev. Sci. Instrum. 87, 063709 (2016) the desired degree of precision and responsiveness. Moreover, it is also possible to move the two motors simultaneously in opposite directions in order to maintain the constant 110 ml/min flow, if that is an important parameter for a particular measurement. An example of open-loop operation is shown in Fig. 4, with the relative humidities in black/orange and temperatures in red/blue plotted as a function of time for both the injection and exhaust chambers, respectively. These measurements demonstrate that the humidity controller can be operated for durations of several days with continuously cycled humidity. In this experimental run, we observed an increase in the baseline level of relative humidity at the end of each sweep when the REG_WET is fully closed. This is a result of the incomplete desorption of water in the tubing and the mixing chambers during the descending humidity ramps. Over multiple cycles, this additional water in the system increased the baseline humidity to 40%–50%—but this effect can be counteracted by allowing the system to stabilise longer during the return to low humidities, or by the use of higher flow elements to improve desorption rates. Finally, the difference between the injected and exhausted humidities can be explained by the large load on the system composed of the atomic force microscope fluid cell as well as over two meters of tubing. III. ATOMIC FORCE MICROSCOPY IMAGING In order to assess the practicality of the humidity controller for real atomic force microscopy measurements, low-amplitude AM-AFM was performed on a flake of highly oriented pyrolytic graphite (HOPG). The flake was first pasted with silver paint onto a standard metallic sample holder. It was then inserted in the AFM fluid cell, micromechanically cleaved in a pure nitrogen flow, and heated for several minutes to 120 ◦C at ∼5% RH in order to partially remove the remaining surface water. An initial increase in exhaust humidity at MIX_EXH was observed during this process, followed by a FIG. 5. Selected images from continuous measurement of ice layer formation on HOPG, illustrating the growth of ice layers. Each image is 3 × 3 µm2. The top row is the topography, the middle row is the phase, and the bottom row is the amplitude signal. Ice layers can be seen growing laterally on the surface, with the most visible contrast in the phase images. The thickness of this ice is ≈0.5 nm, as measured from the topographic images. 063709-5 Gaponenko et al. return to lower values, indicating the efficient water desorption from the graphite. Then, after a short initial stabilisation period, the atmosphere in the cell was almost fully saturated with water to ∼85% RH. The temperature was then lowered from 22 ◦C to −7 ◦C at constant input humidity measured at MIX_INJ. A drop in the exhaust humidity at MIX_EXH was observed, characteristic of condensation processes. When the temperature at the sample had stabilised, continuous amplitude modulation AFM measurements were performed on a clean area of the sample.24 The formation of an ice layer was observed in the form of thin nanoscale layers growing in size as a function of time. The dynamics of the process were imaged by continuous high speed AFM scans acquired for an area of 3 × 3 µm2, with a speed of 256 s/scan. The growth of the ice layer, shown in Fig. 5, is primarily lateral, and is concentrated around nucleation sites distributed randomly in the scan. The thickness of the ice structures formed is measured to be ∼0.5 nm. Moreover, the growth seems to be constrained by the topographic features, with propagation stopping at graphitic step edges. These results are in accordance with the hydrophobic nature of freshly cleaved HOPG, and with the previous studies of icing on this material,25 where comparable thickness layered growth was observed with similar edgelimited dynamics. We note that throughout these measurements we were able to maintain a high subnanometer resolution, with extremely low mechanical nose, as can be seen in both the topography and amplitude images. Such performance was not possible with the bubbler as the source of saturated gas, where the topography signal in similar measurement was unexploitable—leaving only the phase contrast in order to distinguish the growth of the ice layers.20 IV. CONCLUSION In this paper we have described a novel continuous flow humidifier design for noise-critical applications, with a particular focus on atomic force microscopy. We have shown that with the motorised control, relative humidity can be varied continuously over the full range with good finesse and stability. Moreover, we have introduced a dual sensor design which allows the possibility of monitoring more complex water-mediated processes such as condensation and icing by comparing water content of flows injected into and exhausted from the experimental cell. Comparative tests of noise measurements with this atomiser-based design have demonstrated its clear advantage over the mechanically far more noisy bubbler-based system, allowing an imaging resolution comparable to that of a closed cell, or a cell under dry flow only. Finally, we were able to observe the formation of ice on HOPG at low temperatures during continuous wet gas flow, showing extremely low noise levels and high stability of the system over time-dependent measurements. Our system paves the way towards further nanoscale studies on water-sensitive materials, such as those in the semiconductors industry, or towards the investigation of Rev. Sci. Instrum. 87, 063709 (2016) biological processes under various environmental conditions. For the latter, the computer controlled design allows for continuous measurements as a function of humidity without adding potentially stressful external perturbations to the system under measurement as done previously.8,9 Finally, the flexibility of our design does not limit it to the combination of nitrogen and water, and different gases and liquids could be used in order to study their interaction with materials—for example, to probe in real time the response of bacterial growth dynamics to the surrounding environment.26,27 ACKNOWLEDGMENTS This work was supported by the Swiss National Science Foundation under Division II Grant No. 200021-153174. The authors also thank A. Verdaguer and A. Gruverman for helpful discussions. K.H. and L.G. acknowledge the MaNEP student internship programme. 1A. Verdaguer, G. M. Sacha, H. Bluhm, and M. Salmeron, Chem. Rev. 106, 1478 (2006). 2G. Rubasinghege and V. H. 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Temperatures can be measured beween −40 ◦C and 123.8 ◦C, with a precision of ±0.3 ◦C. 20See supplementary material at http://dx.doi.org/10.1063/1.4954285 for a detailed discussion on passive humidification devices, the bill of materials and HOPG icing imaged with a bubbler mechanism. 21Sample provided by Christian Weymann at the Department of Quantum Matter Physics, University of Geneva. The growth was performed by off-axis RF magnetron sputtering with the following conditions: LaNiO3 back electrode 510 ◦C 180 mTorr 10:35 O/Ar 50 W, PbTiO3 (target with 1060 W). 22Measurements were performed with a ASYELEC-01 tip (42.4 nm/V sensitivity, 1.81 N/m spring constant) in amplitude modulation AFM with a free amplitude of 100 mV and a setpoint of 60 mV, corresponding to 063709-6 Gaponenko et al. calibrated amplitudes of 4.2 nm and 2.5 nm, respectively. The 1 µm x 1 µm area was scanned at three lines per second, with a 512 × 512 pixel resolution. 23Available flow elements for the Parker VCD-1000 variable constant differential flow controller span full scale flow rates from 5 to 1500 sccm, a range that can be adapted to the volume of the environmental cell under study. 24Measurements were performed with a ASYELEC-01 tip (42.4 nm/V sensitivity, 1.81 N/m spring constant) in AM-AFM with a free amplitude Rev. Sci. Instrum. 87, 063709 (2016) of 100 mV and a setpoint of 50 mV, corresponding to calibrated amplitudes of 4.2 nm and 2.1 nm, respectively. The 3 µm × 3 µm area was scanned at one line per second, with a 256 × 256 pixel resolution. 25Y. Zheng, C. Su, J. Lu, and K. P. Loh, Angew. Chem., Int. Ed. 52, 8708 (2013). 26U. Herbert, S. Rossaint, M. Khanna, and J. Kreyenschmidt, Poult. Sci. 92, 1348 (2013). 27J. L. Etchells, T. A. Bell, R. N. Costilow, C. E. Hood, and T. E. Anderson, Appl. Microbiol. 26, 943 (1973).