Graphene encapsulation enabled high-throughput atom probe tomography of liquid specimens
Graphical abstract
Overview of the proposed method of preparing liquid specimens for atom probe tomography. (a) Schematic of encapsulating liquid on a pre-sharpened tip using graphene, with controlled encapsulation speed and number of encapsulations. (b) APT imaging of the graphene-encapsulated liquid specimen in laser-pulsed mode. (c) Acquired mass-to-charge-state ratio spectrum and the reconstructed 3D chemical map of the field-evaporated liquid (water) volume on base W tip with near-atomic spatial resolution.
Introduction
Atom probe tomography (APT) originates from the coupling of field ion microscopy (FIM) with a time-of-flight mass spectrometer [1], and is considered as the only approach to provide both structural and chemical information with near-atomic resolution (~0.3 nm laterally and ~0.1 nm in depth) [2]. This technique has recently demonstrated its unique capability of investigating a wide range of materials [3], [4], [5], [6]. During APT imaging, an electrostatic field is generated by the application of high voltage on a needle-shaped specimen maintained under ultra-high vacuum and cryogenic conditions. Controlled high-voltage or laser pulses superimposed to the electrostatic field enables evaporation of field-ionized atoms and molecules from the surface of the specimen tip to a single-particle and position-sensitive detector. Processing the data from the detector includes the translation of ion time-of-flight to mass-to-charge state ratio, which determines the ionic species, and the use of a reverse projection algorithm to reconstruct a three-dimensional (3D) chemical map at near-atomic spatial resolution based on the impact position and sequence of each ion [7], [8], [9], [10]. In order to achieve an electrostatic field suitable for ionization, the end radius of the specimen is required to remain less than ∼75 nm [11]. Conventional specimen preparation technique by focused ion beam (FIB) lift-out method, however, is performed in vacuum and limits APT imaging to solid specimens only [12]. Damage induced by ion implantation during the FIB milling is almost inevitable for biological materials [13, 14]. Preparing APT specimens in their hydrated state has been a longstanding challenge, and one solution has been to develop cryo-transfer accessories [15]. This allows a direct transfer of the frozen APT specimens, prepared by cryo-FIB lift-out method, to the cryogenic stage in the atom probe instrument. However, this approach requires significant cryo-chain instrumentation including cryo-FIB-SEM, the atom probe instrument and UHV/cryo-transfer accessories, as well as significant time to prepare a single APT specimen [15, 16].
In the present study, a new method of preparing liquid specimens for APT has been demonstrated by encapsulating the target solution on pre-sharpened specimen tips using single graphene membranes. The volume of encapsulated liquid can be tuned by the encapsulation speed and the number of encapsulations, to achieve an end radius of the final graphene-encapsulated specimen suitable for APT experiments. Subsequently, the liquid specimen is inserted into the atom probe instrument and then mounted on the cryogenic stage where it is maintained in the frozen hydrated state, ready for laser-pulsed APT. In this study, the graphene-encapsulation method has been proven feasible firstly with water (ice) on a tungsten (W) specimen tip, followed by ferritin in heavy water on a silicon (Si) specimen tip. In the latter case, the method is applied to a Si micro-tip array to produce multiple liquid specimens at the same time, laying the foundation for high-throughput APT imaging. The graphene encapsulation method overcomes the challenge of applying APT to hydrated specimens, allowing APT investigation of various materials in a liquid environment.
Section snippets
Specimen preparation
In the initial preparation stage, W tips (American Probe & Technologies, Merced, CA, USA, Product number: 72X-G3/01) and Si micro-tip array on wafer (Melbourne Center for Nanofabrication) were pre-sharpened using FIB with a gallium ion (Ga+) source on a FIB/SEM system (FEI Quanta 3D system, Thermo Fisher, USA). Specifically, the micro-tip array consists of 36 Si micro-tips on a Si wafer, and each micro-tip has been fabricated with a height of 150 ± 50 µm, bottom diameter of 30 ± 10 µm and apex
Graphene encapsulation of water on tungsten (W) tips
In a previous report, graphene has been employed to encapsulate dehydrated biological specimens in order to enhance the electrical conductivity for voltage-pulsed APT [18]. In the current study, graphene encapsulation method has been extended for preparing APT specimens in the hydrated state. Graphene encapsulation of water on W tips was achieved with the customized experimental setup as shown in Fig. 1a, and details of the procedure are presented in Fig. 1b-d. A single graphene membrane was
Conclusion
An innovative method of preparing APT specimens in their hydrated state has been presented, which overcomes the longstanding challenge for APT imaging of hydrated specimens. By introducing graphene encapsulation, liquid can be effectively trapped on top of pre-sharpened specimen tips. The volume of liquid can be controlled, and the final geometry of graphene-encapsulated liquid specimen remains suitable for field ionization and evaporation. Using water as an example, the prepared specimens have
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was partly funded by the Australian Research Council (DP180103955), Monash University Interdisciplinary Research (IDR) Seed Fund and the Monash Centre for Atomically Thin Materials (MCATM) (for Ph.D. top-up scholarship). This work was performed in part at the Melbourne Centre for Nanofabrication (MCN), Victorian Node of the Australian National Fabrication Facility (ANFF). Also, the authors acknowledge the use of facilities within the Monash Centre for Electron Microscopy (MCEM),
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