High-stability Quartz-crystal Microbalance Research Articles

High Sensitivity and High Stability QCM Humidity Sensors Based on Polydopamine Coated Cellulose Nanocrystals/Graphene Oxide Nanocomposite.

In this paper, a high sensitivity and high stability quartz crystal microbalance (QCM) humidity sensor using polydopamine (PDA) coated cellulose nanocrystal (CNC)/graphene oxide (GO) (PDA@CNC/GO) nanocomposite as sensitive material is demonstrated. The PDA@CNC was prepared by the self-polymerization action on the surface of CNC, and it acted as filler material to form functional nanocomposite with GO. The material characteristics of PDA@CNC, CNC/GO and PDA@CNC/GO were analyzed by transmission electron microscope (TEM) and Fourier transform infrared spectroscopy (FTIR), respectively. The experimental results show that the introduction of PDA@CNC into GO film not only effectively enhanced the sensitivity of GO-based nanocomposite-coated QCM sensor but also significantly maintained high stability in the entire humidity range. The PDA@CNC/GO30-coated QCM humidity sensor exhibited a superior response sensitivity up to 54.66 Hz/% relative humidity (RH), while the change rate of dynamic resistance of the sensor in the humidity range of 11.3–97.3% RH is only 14% that is much smaller than that of CNC/GO-coated QCM. Besides, the effect of the PDA@CNC content on the sensitivity and stability of GO-based nanocomposite-coated QCM humidity was also studied. Moreover, other performances of PDA@CNC/GO-coated QCM humidity sensor, including humidity hysteresis, fast response and recovery and long-term stability, were systematically investigated. This work suggests that PDA@CNC/GO nanocomposite is a promising candidate material for realizing high sensitivity and high stability QCM humidity sensor in the entire humidity detection range.

Using a high-stability quartz-crystal microbalance to measure and model the chemical kinetics for gases in and on metals: Oxygen in gold

This paper describes the use of a high-stability quartz-crystal microbalance (QCM) to measure the mass of a gas absorbed on and in the metal electrode on the quartz oscillator, when the gas pressure is low and the gas can be considered as rigidly attached to the metal, so the viscosity effects are negligible. This provides an absolute measure of the total mass of gas uptake as a function of time, which can be used to model the kinetic processes involved. The technique can measure diffusion parameters of gases in metals close to room temperature at gas pressures much below one atmosphere, as relevant to surface processes such as atomic layer deposition and model studies of heterogeneous catalysis, whereas traditional diffusion measurements require temperatures over 400 °C at gas pressures of at least a few Torr. A strong aspect of the method is the ability to combine the “bulk” measurement of absorbed mass by a QCM with a surface-sensitive technique such as Auger electron spectroscopy in the same vacuum chamber. The method is illustrated using atomic oxygen, formed under O2 gas at 6 × 10−5 Torr in the presence of a hot tungsten filament, interacting with the gold electrode on a QCM crystal held at 52 to 120 °C. Some of the incident oxygen forms a surface oxide which eventually blocks more uptake, and the rest (about 80%) indiffuses. Surprisingly, the rate of oxygen uptake initially increases with the amount of oxygen previously absorbed; therefore, the measured oxygen uptake with time is reproducible only if preadsorption of oxygen conditions the sample. Temperatures above 130 °C are necessary for measurable thermal desorption, but all the oxygen can be removed by CO scavenging at all temperatures of these experiments. Simple kinetic models are developed for fitting the experimental QCM data to extract parameters including those controlling adsorption of oxygen, the CO scavenging probability, and limits on the diffusion jump frequency of dissolved oxygen. The reproducibility of the data and the good model fits to it provide proof-of-principle for the technique.

High-stability quartz crystal microbalance ammonia sensor utilizing graphene oxide isolation layer

This paper presents a new sensing structure to enhance the stability of a quartz crystal microbalance (QCM) ammonia sensor. The sensing structure consists of bi-layer films, in which graphene oxide (GO) film serves as an isolation layer between the electrode of the QCM and polyaniline (PANI) sensing film. The quality (Q) factor of the QCMs was measured with and without the GO isolation layer. The experiment results indicate that the quality factor of QCM with GO layer is about two times larger than that of the QCM without GO in the whole detection range, which means the introduction of GO isolation layer can effectively improve the quality factor of the QCM. The improvement of the Q factor is ascribed to the high elasticity modulus of GO isolation layer, which produced the suppression of the surface energy loss of the QCMs. The use of GO as isolation layer provides a simple way to realize high stability QCM sensors.

Investigation of the stability of QCM humidity sensor using graphene oxide as sensing films

In this paper, the stability of quartz crystal microbalance (QCM) humidity sensor based on graphene oxide (GO) films is investigated by impedance analysis method. The stability characteristic of GO based QCM sensor during humidity detection is studied and quantitatively compared with conventional polymer coated QCM sensor. It is found that the stability of GO based QCM sensor is significantly higher than that of polymer coated QCM sensor at any humidity condition. Especially at high humidity points, the advantage of the stability of GO based QCM sensor becomes more prominent, the quality factor of GO based QCM sensor is nearly 25 times greater than that of polymer based QCM sensor. Further, a possible mechanism of the experimental phenomena is suggested. This research provides a simple and effective approach to construct high stability QCM chemical sensor by using GO based materials as the sensing layer.

Stabilization of sample temperature in a surface-science vacuum chamber to 0.03 K and quartz-crystal microbalance frequency to 0.06 Hz over 0.5 h

Improvements have been made to a high-stability quartz-crystal microbalance for use in a typical surface-science, ultrahigh vacuum chamber, with a frequency stability of one part in 10(8) (0.06 Hz) over 0.5 h. This gives a resolution equivalent to 2% of an atomic monolayer of oxygen over 0.5 h. The quartz-crystal microbalance (QCM) crystal can be rotated to different surface-analysis positions in the chamber. These characteristics open up the combination of surface and bulk adsorption studies on the same sample without transferring the sample to another chamber. To accomplish this, it was necessary to stabilize the sample temperature to ±0.03 K over several hours. The oscillator performance is illustrated by the uptake of oxygen by a gold-plated QCM crystal.

Ultrathin films of lead oxide on gold: Dependence of stoichiometry, stability and thickness on O 2 pressure and annealing temperature

Ultrathin oxide films grown in vacuum are important in many industrial areas, including microelectronics and heterogeneous catalysis. In this paper, the dependence of oxide stoichiometry, growth kinetics, thickness and stability on O 2 pressure and annealing temperature are explored using a high-stability quartz-crystal microbalance and Auger spectroscopy, for the oxidation of lead on gold as a model system. The oxide thickness increases abruptly at specific values of the O 2 pressure, as explained previously using Gibbs free energies. A qualitative difference is found between lead-oxide films which are 1 monolayer thick and those which are 2 or more monolayers thick; the former apparently involve exclusively chemisorbed oxygen and can be oxidized and reduced reversibly using thermal oxidation/annealing cycles, whereas the latter involve an extended lead oxide, are more thermally stable, and have a smaller electron inelastic mean free path. Accurate values of the O 2 sticking probability are obtained.

Dependence of oxide thickness on O2 pressure and experimental determination of Gibbs free energies of ultrathin oxide films in vacuum

Ultrathin oxide films grown in a vacuum are important in many industrial processes, with the values of Gibbs free energies of the gas and films determining the oxide type that grows on a surface, and its thickness. A high-stability quartz-crystal microbalance is used to provide quantitative experimental data on these free energies, for the model case of oxidation of a lead film on a gold substrate. The surface oxide, PbO, forms a single molecular layer at 10−6Torr of O2, but thickens abruptly to two layers at 1×10−4Torr, and becomes even thicker by 0.33Torr. This is explained using film free energies.

High-stability quartz-crystal microbalance for investigations in surface science

This article describes a high-stability quartz-crystal microbalance (QCM) and the methodology for measuring the change in mass during thin-film growth in deposition and sputter processes. Much lower noise and higher-frequency stability have been achieved than with conventional QCMs. A stability of ±0.1 Hz at 6 MHz has been obtained over 4 h, with a rms stability of 0.03 Hz. The adsorption of one atomic monolayer of oxygen produces a frequency shift of about 5 Hz, so this stability enables the QCM to be used to determine the stoichiometry of submonolayer oxide films, as well as for high-accuracy measurements of adsorbate sticking probability and ion-milling rate.

Absolute determination of the stoichiometry of ultra-thin oxide films as a function of thickness: antimony oxide on gold

Oxide layers only a few atomic layers thick are important in electronic devices and heterogeneous catalysis. This has led to the need to understand better the evolution of the oxide stoichiometry with thickness in such situations. This paper shows that a high-stability quartz-crystal microbalance can provide absolute measurements of changes in the stoichiometry of oxides as they evolve one molecular layer at a time. In the case of ultra-thin antimony oxides grown on a gold substrate, the compound SbAuO was observed for Sb deposits of less than 1.6 atomic layers. This oxide acts as a transition layer between the gold and subsequent oxide layers which are Sb2O3.