Control of Gallium Oxide Growth on Liquid Metal Eutectic Gallium

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Control of Gallium Oxide Growth on Liquid Metal Eutectic Gallium/Indium Nanoparticles via Thiolation Zachary J Farrell, and Christopher Tabor Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03384 • Publication Date (Web): 07 Dec 2017 Downloaded from http://pubs.acs.org on December 17, 2017

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Control of Gallium Oxide Growth on Liquid Metal Eutectic Gallium/Indium Nanoparticles via Thiolation Zachary J. Farrell†,‡ and Christopher Tabor∗,‡ †UES, Inc. ‡Air Force Research Laboratory, Dayton, OH E-mail: [email protected]

Abstract Eutectic gallium-indium alloy (EGaIn, a room-temperature liquid metal) nanoparticles are of interest for their unique potential uses in self-healing and flexible electronic devices. One reason for their interest is due to a passivating oxide skin that develops spontaneously on exposure to ambient atmosphere which resists deformation and rupture of the resultant liquid particles. It is then of interest to develop methods for control of this oxide growth process. It is hypothesized here that functionalization of EGaIn nanoparticles with thiolated molecules could moderate oxide growth based on insights from the Cabrera-Mott oxidation model. To test this, the oxidation dynamics of several thiolated nanoparticle systems were tracked over time with x-ray photoelectron spectroscopy. These results demonstrate the ability to suppress gallium oxide growth by up to 30 percent. The oxide progressively matures over a 28 day period terminating in different final thicknesses as a function of thiol selection. These results indicate that not only do thiols moderate gallium oxide growth via competition with

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oxygen for surface sites, but that different thiols alter the thermodynamics of oxide growth through modification of the EGaIn work function.

Introduction In recent years, eutectic gallium-indium alloy (EGaIn) has sparked the interest of researchers due to its unique material properties. This alloy, which is liquid at room temperature (14.2 at% In, 85.8 at% Ga, melting point = 10 ◦ C), is inherently flexible/stretchable, has a viscosity near that of water, has high electrical and thermal conductivity, alloys readily with most metals, is non-toxic, and spontaneously forms a self-passivating gallium oxide layer on exposure to ambient atmosphere. Consequently, it has already found utility in flexible circuits and interconnects, antennas, conformal electrodes, and self-repairing circuitry. 1–8 Although basic implementations of the aforementioned applications have been produced, scientific understanding of how best to harness this material system for various engineering applications is still relatively immature. A major area of concern relates to the interfacial chemistry of EGaIn, specifically to the growth and presence of the native gallium oxide layer. Under ambient conditions, this layer modifies some of the observed material properties of EGaIn in the bulk; for instance, EGaIn is a Newtonian liquid with a viscosity near water (2 mP a ∗ s), however the flow behavior resembles that of a thick paste due to the viscoelastic gallium oxide. 9 The thickness of this oxide layer has been measured as being anywhere between 0.5-5 nm thick in the bulk, depending somewhat on growth conditions. 1,10–16 The oxide plays an increasingly important role in the properties of colloidal EGaIn; as the surface/volume ratio increases drastically, an ever greater fraction of the nanoparticles are taken up by this oxide layer. 14,17,18 As many interesting applications (self-healing circuitry, mechanical joining of printed EGaIn colloids, etc.) rely on rupturing or removing the native gallium oxide shell, it is imperative to develop techniques for controlling the thickness of any oxide shell present. 5 Based on observations from Hohman, et al. in which thiolated

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molecules were used to assist in division of bulk EGaIn into nanoparticles via ultrasonication (and changes in surface roughness of the resulting particles were observed in SEM), it was hypothesized that thiolated molecules could potentially inhibit gallium oxide growth, although a full description of the mechanism of oxide formation was not presented. 15 One likely mechanism that can account for oxide growth on gallium alloys is the CabreraMott oxidation mechanism. This theory of oxidation has been shown to describe the oxidation process of nanoparticles of other group III metals, such as indium and aluminum as well as some bulk gallium-containing semiconductors. 19–21 In this framework, the process of oxide growth is driven by an electric field which is established by the adsorption of O2 on the particle surface, creating new surface states (equal to the O2p energy level, situated between the Fermi level of the metal and the valence level of the oxide) in the process. Thus, the strength of the established electrical potential is equivalent to the difference between the metal work function and the O2p energy level (relative to vacuum). 22 Additionally, certain implementations of Cabrera-Mott attempt to account for the availability of surface sites on which O2 can adsorb and dissociate; in general, there is a direct proportionality between the number of available surface sites and the rate of oxidation. 23 It is well-known that adsorbed molecules such as thiols can influence the measured work function by modifying the perceived vacuum level via their molecular dipole, and are also expected to change the dynamics of O2 adsorption by competition for surface sites. Based on this, we hypothesize here that both the work function modification and the competition for surface sites will lead to observed differences in gallium oxide growth, based on the Cabrera-Mott mechanistic framework described above. 24–26 Three thiolated molecules were chosen for experimentation (in addition to the unfunctionalized control case): dodecanethiol (DDT), thiophenol (TP), and 2,3,4,5,6-pentafluorothiophenol (FTP). DDT is expected to provide a similar molecular dipole to TP while varying surface coverage, thus changing the amount of O2 restriction to the surface. 27–29 Additionally, while TP and FTP should occupy relatively the same surface coverage, their effect on the work function of the metallic core

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should be drastically different from one another. 26 Due to conjugation of pi electrons with electrons in the p-orbital of the sulfur (in the bonding thiol headgroup) of TP and FTP, these molecules are expected to be able to delocalize electrons into or out of the (elemental) metal surface depending on the dipole direction, leading to a respective decrease or increase in work function. 26,30,31 STEM images of particles functionalized with these molecules, as well as molecular structures and dipoles of the various ligands are illustrated in Figure 1.

Figure 1: STEM images of each of the four nanoparticle samples studied paired with their respective functionalizing molecules and the strength of the molecular dipoles. DDT and TP are expected to decrease the work function based on their dipole direction, while FTP is expected to increase it. Through this approach, we demonstrate here that thiolation of EGaIn nanoparticles can control the overall oxide thickness by controlling the dynamics of oxide growth through modification of the degree of access for O2 molecules as well as by exerting thermodynamic control through alteration of the work function of the metal core (and thus the electrical potential driving oxidation). 4


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Experimental Materials 99.9999% purity Ga shot and 99.9999% purity In shot were purchased from Indium Corporation and used to produce EGaIn (14.2 at% In, 85.8 at% Ga) as needed. 1-dodecanethiol (≥98%), thiophenol (≥99%), 2,3,4,5,6-pentafluorothiophenol (≥97%), and chlorobenzene (ReagentPlus, ≥99%) were purchased from Sigma-Aldrich and used as received.

Fabrication of EGaIn Nanoparticles Unless otherwise specified, all subsequent work was performed inside a glovebox under inert atmosphere containing < 1 PPM of O2 and < 5 PPM H2 O. All solvents were degassed by 3x freeze-pump-thaw cycles. 0.1 mL of EGaIn was transferred via autopipette into a standard 20 mL, 28 mm OD borosilicate scintillation vial containing 14.9 mL of either a 64 mM solution of a thiolated molecule in chlorobenzene (dodecanethiol, thiophenol, 2,3,4,5,6-fluorothiophenol) or pure chlorobenzene. These vials were socketed into a custom-built aluminum jig block mated to a TETech CP-121 Peltier cold plate maintained at 10 ◦ C (to counter solvent vaporization from sonication induced heating). A 3mm tapered microtip powered by a Sonics and Materials, Inc. VCX 500 ultrasonic processor was then immersed into the open vial. The opening between the vial and probe microtip was covered as completely as possible with Parafilm to aid in solvent retention. The VCX-500 was then set to an amplitude of 20% and a sonication time of 2 hours. Immediately after beginning ultrasonication, the liquid in the vial was observed to quickly darken, indicating the formation of EGaIn particles. The formation of gallium oxide was inferred to still occur even under these low O2 conditions, as recoalescence of particles (forming bulk EGaIn) would otherwise be observed for the control sample. 1 In thiolated samples, oxide growth is expected to originate from defect sites in the thiol monolayer, as has been previously observed in copper. 32 After sonication, particle suspensions were capped and removed from the glovebox. To 5


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recover purified particle suspensions, the as-formed suspensions were centrifuged at 800 RPM for 10 minutes and the supernatant decanted and replaced with fresh (non-degassed) chlorobenzene. This process was repeated a total of three times to remove any unbound thiols from solution. The final suspensions were stored capped under ambient conditions.

STEM Particle Sizing and Gallium Oxide Thickness Measurement Samples were prepared for TEM by deposition of particle suspensions onto ultrathin (3-4 nm) carbon films on 400-mesh copper grids purchased from Electron Microscopy Sciences (CF400-Cu-UL). Prior to deposition, 1-2 drops of the purified final suspensions were added to 2 mL of dichloromethane. This dilute suspension was then dropped onto a TEM grid held in self-closing, anti-capillary tweezers until a single drop fell from the grid. A folded piece of filter paper was then used to wick excess solvent from the grid underside. Any samples prepared for gallium oxide shell thickness measurement were measured within 1 hour of sample preparation. High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images collected on an FEI Talos transmission electron microscope operating at 200 keV were used for particle sizing measurements. As particles tended to agglomerate during drying, degrading contrast between individual particles and interfering with image processing, automated routines were not used for particle sizing. Size measurements were made manually with at least 400 particles counted per sample. For calculation of gallium oxide shell thickness for use in benchmarking XPS measurements, HAADF images were processed using the “Find Edges“ function in ImageJ. This routine utilizes a Sobel filter to calculate an image in which pixel intensity represents (in the case of HAADF images) the rate of atomic number change as a function of position. 33 As there is a large atomic number difference between the different species making up the particle shells and core (Z C = 4, Z GaOxide = 17.2, Z EGaIn = 34.07), intensity peaks are produced at particle-shell interfaces which represent the point of maximum atomic number change with 6


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distance. Two peaks are typically formed; one at the organic-gallium oxide interface and one at the gallium oxide-EGaIn interface. The distance between these peaks can then be taken as a relatively objective measure of gallium oxide thickness. Conceptually, it may also be possible to resolve a third peak at the exterior of the carbonaceous particle shell, however, as the TEM grid has a continuous carbon film, this will not be visible unless the film material is changed to a material other than carbon.

XPS Measurements X-ray photoelectron spectroscopy (XPS) measurements were taken of EGaIn nanoparticle films over a period of 28 days to measure gallium oxide growth. Films were prepared 30 minutes prior to introduction into the the XPS on each measurement day by spin-coating at 3000 RPM a small amount of each suspension to be measured onto copper foil affixed to a 1 cm x 1 cm piece of glass. Suspensions were vortex mixed prior to deposition to ensure homogeneity and deposition was carried out until the film was visibly opaque. The XPS measurements were carried out on an SSI M-Probe with all samples to be measured loaded at once and kept under ultra-high vacuum for the duration of the measurement. For all samples, the measurements and regions were as follows: survey spectrum, high-resolution spectrum 30 eV wide centered at 19 eV BE (Ga3d), high resolution spectrum 30 eV wide centered at 445 eV BE (In3d), high resolution spectrum 20 eV wide centered at 532 eV (O1s), and a high resolution spectrum 20 eV wide centered at 1117 eV (Ga2p). The Ga3d region background was fitted with a Tougaard universal background, O1s and Ga2p backgrounds with a Shirley, and the In3d with linear. All peaks were fitted with Gaussian-Lorentzian line shapes and all measurements were taken with the same number of scans to permit simple comparison between regions. More detailed information on peak fitting procedure, error determination, and reproduction of all spectra may be found in the supporting information.

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Results and discussion EGaIn nanoparticle suspensions were characterized using STEM to get measurements of the particle size distribution. Representative STEM images and the derivative particle size distributions are shown as Figures 1 (appearing earlier) and 2, respectively.

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Figure 2: Particle size distributions for the samples corresponding to the STEM images in Figure 1. All samples show the heavily skewed log-normal distribution expected for top-down production of particles. As can be seem from the above images and histograms, the distribution of nanoparticle sizes is approximately log-normal; this is typical for particles formed in a top-down manner (rather than the Gaussian distribution expected for a bottom-up synthetic approach). 34,35 At very long sonication times, the particle size distribution eventually becomes more monodisperse, converging to distributions similar to that seen by Lear, et al at 66 hours of sonication. 14 A representative STEM image and histogram are shown in the supplementary information as Figures S30 and S31. 8


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Due to the extreme variance in measured gallium oxide thickness in the literature, it was of interest to choose measurement techniques which would provide a more precise method of gallium oxide thickness determination than the brightfield TEM contrast approach which is common in the literature. 36,37 A possible method which could reduce this subjectivity is x-ray photoelectron spectroscopy (XPS). 16,36 As maximum photoelectron escape depth is typically estimated at ∼10 nm, photoelectrons will penetrate the gallium oxide shell in all cases, but will not penetrate through entire nanoparticles. 38 Thus, tracking the fraction of gallium(III) vs. total gallium via XPS gives a straightforward measure of the relative degree of gallium oxide thickness. A comparison between the thiolated EGaIn nanoparticles (DDT, TP, FTP) and the control (bare) over the 28 day measurement cycle appears as Figure 3.

Figure 3: (A) Representative XPS measurements for the Ga3d region at 1 and 28 days after colloid production. Peaks of interest are indicated with arrows. Signals originating from Ga(0) and In(0) are observed to drastically decrease relative to the Ga(III) signal over the 28 day span. (B) Relative concentration of Ga(III) vs. total Ga as a function of time and thiolating species based on quantification of the Ga3d peak. All thiolated species show a significant reduction in Ga(III) concentration, indicating a thinner gallium oxide shell. FTP has the lowest measured final ratio of Ga(III) to total Ga, indicating the thinnest gallium oxide shell. The ordering of thiolated species corresponds to expected results based on molecular dipole direction and strength.

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Although the results shown in Figure 3B do exhibit the hypothesized reduction in gallium oxide thickness as well as the hypothesized ordering in oxide reduction of the tested thiols, they do not directly convey any information about the absolute thickness of either the bare or thiolated EGaIn nanoparticle gallium oxide shells. The next section details a method for converting these relative measurements into absolute numbers. Based on the work of Cant, et al., Shard, and Seah, it is possible to obtain absolute thickness measurements from the above data with a minimal set of assumptions. 39–41 The first step implicit in this process is to model the functionalized EGaIn nanoparticles as core-shell-shell particles, where the core is assumed to be composed of EGaIn (14.2 at% In, 85.8 at% Ga), the inner shell is composed of gallium oxide (assumed stoichiometric Ga2 O3 ), and the outer shell is organic (assumed Z = 4 based on Seah). 42 This layout is schematically depicted as panel (a) of Figure 4. Additionally, particle sizes for all samples were measured via STEM and weighted using a surface-area average particle size, to account for polydispersity; this approach is similar to that implemented by Shard in calculation of shell thicknesses in core-shell particles via XPS. 40 To translate XPS intensity ratios into thicknesses, photoelectron attenuation lengths were calculated for one of the layers. The relationship used for this purpose is the well-known equation developed by Cumpson and Seah shown as Equation 1 below ( L = 0.316a

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where the attenuation length in nm, L, is calculated as a function of the kinetic energy of the photoelectron of interest in eV, E, the atomic length in nm, a, and the average atomic number, Z. 41 Although Z may be calculated to a reasonable degree of confidence from knowledge about the initial material composition and the most stable gallium oxide species, a is less straightforward to quantify due to the fact that as formed gallium oxide in this system is amorphous. 43 As a result, estimates of a based on crystalline forms of gallium oxide (aβGa

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estimates for a for benchmarking derivative XPS measurements, STEM images were taken for TP-functionalized EGaIn nanoparticles at 1 day and again at 28 days; these images were processed as detailed in the Experimental section to reveal gallium oxide shell thicknesses of 1.54 and 2.40 nm, respectively. Before and after images, as well as a line profile which reveals the two expected peaks (corresponding to the carbon-gallium oxide and gallium oxide-EGaIn interfaces) are shown below as panels (b)-(d) of Figure 4.

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Figure 4: (a) Schematic showing the core-shell-shell model of an EGaIn nanoparticle (layers not to scale). Additionally, it is likely that the gallium oxide shell is not totally continuous (as depicted) in thiolated nanoparticles as thiols are expected to bind to elemental gallium rather than gallium oxide.(b) and (c) Representative HAADF image of 1 day old TP-functionalized EGaIn nanoparticles before and after (contrast in (b) inverted to highlight edges) processing with a Sobel filter to reveal gallium oxide shell thickness. (d) Line profiles were taken perpendicular to the particle surface through the shell to easily assess intensity peaks. Average gallium oxide shell thickness were: hT P,1day = 1.23 ± 0.20 nm and hT P,28days = 2.12 ± 0.26 nm.

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As an independent measurement of gallium oxide thickness had been made via STEM, a simple implementation of the iterative method for shell thickness calculation from XPS laid out by Cant et al. was produced for MATLAB. By minimizing the sum of squares error difference between the XPS measured thickness and the HAADF STEM measured thickness on days 1 and 28, the unknown gallium oxide atomic length was calculated to be a = 0.19 nm. This value is well within the range of atomic sizes expected for oxides as given by Seah and has surprisingly good correspondence to the values for aβGa

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, 0.18 and 0.20 nm. 42,44

If the XPS data used to produce the relative thickness measurements shown in Figure 3 are reprocessed via this algorithm, it is possible to produce a plot of absolute gallium oxide thickness vs. time; this graph appears as Figure 5.

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As indicated in Figure 5, all thiolated samples show a thinner gallium oxide shell than the control (bare, unfunctionalized) sample. At early times, thiolation suppresses oxide growth by up to at 30% vs. the control, while at 28 days it maintains a 17 % reduction. This is a highly significant modification, as the force required to rupture the gallium oxide shell is expected to increase with the square of shell thickness; thus a 17% reduction in thickness is expected to translate to an approximate 31% decrease in bursting force. 17 Additionally, as hypothesized, the FTP sample (expected to increase the metal work function) resists oxidation more than the TP or DDT (expected to decrease work function) samples at long times. Differences in oxide thickness at early times are likely due to differences in ligand coverage or packing, which could provide a physical barrier to oxygen diffusion, thus changing the kinetics of oxide growth but having little impact on the final (thermodynamically-driven) oxide thickness. For further corroboration of these results, STEM measurements were taken for all particle systems at 1 and 28 days which support the general conclusions drawn from XPS, but suffer from a large amount of statistical noise due to the difficulty of taking a large number of measurements; these as well as a replicate data set (4 replicate samples measured with XPS over 28 days) which supports the findings shown in Figure 5 are available in the supplementary information.

Conclusions Thiolation has been used to successfully exert control over the growth of a gallium oxide shell on liquid eutectic gallium-indium nanoparticles, as quantified by the relative reduction in [Ga(III)]/[Ga] ratio measured by XPS as well as absolute thicknesses computed from the same. Via this characterization method, it was demonstrated that thiolation does in fact mitigate (but not eliminate) the growth of a gallium oxide shell vs. a non-thiolated control. Additionally, FTP exhibited the greatest reduction in gallium oxide thickness, as expected based on its dipolar characteristics acting to modify the EGaIn work function. Finally, the

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terminal thickness of 2.13-2.66 nm corresponds to the range of oxide thicknesses previously reported in the literature. Interestingly, however, the oxide formation process takes weeks to reach a stable thickness despite the impression of instantaneous formation of mature gallium oxide conveyed by the existing literature. The novel insights on the colloidal EGaIn system developed in this publication are anticipated to lead to better tailored approaches in engineering materials for self-healing electronics and other flexible/stretchable circuitry as well as any other applications of gallium alloys in which gallium oxidation plays a role in the surface chemistry.

Acknowledgement The authors thank Dr. Adam Waite for his input into XPS spectra deconvolution and interpretation and Arthur Safriet for his design and construction of a jig for Peltier cooling of scintillation vials during ultrasonication.

Supporting Information Available Particle size distributions, additional TEM images, and XPS spectra (with peak fits) are available in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Dickey, M. D. Stretchable and soft electronics using liquid metals. Advanced Materials 2017, 1606425, 1–19. (2) Boley, J. W.; White, E. L.; Chiu, G. T. C.; Kramer, R. K. Direct writing of galliumindium alloy for stretchable electronics. Advanced Functional Materials 2014, 24, 3501– 3507. 15


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(3) Blaiszik, B. J.; Jones, a. R.; Sottos, N. R.; White, S. R. Microencapsulation of galliumindium (Ga-In) liquid metal for self-healing applications. Journal of Microencapsulation 2014, 2048, 2013–2014. (4) Blaiszik, B. J.; Kramer, S. L. B.; Grady, M. E.; McIlroy, D. A.; Moore, J. S.; Sottos, N. R.; White, S. R. Autonomic restoration of electrical conductivity. Advanced Materials 2012, 24, 398–401. (5) Boley, J. W.; White, E. L.; Kramer, R. K. Mechanically sintered gallium-indium nanoparticles. Advanced Materials 2015, 27, 2355–2360. (6) Diebold, A. V.; Watson, A. M.; Holcomb, S.; Tabor, C.; Mast, D.; Dickey, M. D.; Heikenfeld, J. Electrowetting-actuated liquid metal for RF applications. Journal of Micromechanics and Microengineering 2017, 27, 025010. (7) Holcomb, S.; Brothers, M.; Diebold, A.; Thatcher, W.; Mast, D.; Tabor, C.; Heikenfeld, J. Oxide-free actuation of gallium liquid metal alloys enabled by novel acidified siloxane oils. Langmuir 2016, 32, 12656–12663. (8) Ilyas, N.; Cook, A.; Tabor, C. E. Designing liquid metal interfaces to enable next generation flexible and reconfigurable electronics. Advanced Materials Interfaces 2017, 1700141, 6–11. (9) Dickey, M. D.; Chiechi, R. C.; Larsen, R. J.; Weiss, E. A.; Weitz, D. A.; Whitesides, G. M. Eutectic gallium-indium (EGaIn): A liquid metal alloy for the formation of stable structures in microchannels at room temperature. Advanced Functional Materials 2008, 18, 1097–1104. (10) McGuiness, C. L.; Shaporenko, A.; Zharnikov, M.; Walker, A. V.; Allara, D. L. Molecular self-assembly at bare semiconductor surfaces: investigation of the chemical and electronic properties of the alkanethiolate-GaAs(001) interface. The Journal of Physical Chemistry C 2007, 111, 4226–4234. 16


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Graphical TOC Entry

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Control of Gallium Oxide Growth on Liquid Metal Eutectic Gallium

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