Bulk and Thin Film Synthesis of Compositionally Variant Entropy-stabilized Oxides

요약

The synthesis of high quality bulk and thin film (Mg_0.25(1-x)Co_xNi_0.25(1-x)Cu_0.25(1-x)Zn_0.25(1-x))O and (Mg_0.25(1-x)Co_0.25(1-x)Ni_0.25(1-x)Cu_xZn_0.25(1-x))O entropy-stabilized oxides is presented.

초록

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg_0.25(1-x)Co_xNi_0.25(1-x)Cu_0.25(1-x)Zn_0.25(1-x))O (Co variant) and (Mg_0.25(1-x)Co_0.25(1-x)Ni_0.25(1-x)Cu_xZn_0.25(1-x))O (Cu variant) entropy-stabilized oxides. Phase pure and chemically homogeneous (Mg_0.25(1-x)Co_xNi_0.25(1-x)Cu_0.25(1-x)Zn_0.25(1-x))O (x = 0.20, 0.27, 0.33) and (Mg_0.25(1-x)Co_0.25(1-x)Ni_0.25(1-x)Cu_xZn_0.25(1-x))O (x = 0.11, 0.27) ceramic pellets are synthesized and used in the deposition of ultra-high quality, phase pure, single crystalline thin films of the target stoichiometry. A detailed methodology for the deposition of smooth, chemically homogeneous, entropy-stabilized oxide thin films by pulsed laser deposition on (001)-oriented MgO substrates is described. The phase and crystallinity of bulk and thin film materials are confirmed using X-ray diffraction. Composition and chemical homogeneity are confirmed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy. The surface topography of thin films is measured with scanning probe microscopy. The synthesis of high quality, single crystalline, entropy-stabilized oxide thin films enables the study of interface, size, strain, and disorder effects on the properties in this new class of highly disordered oxide materials.

서문

Since the discovery of high-entropy metal alloys in 2004, high-entropy materials have attracted significant interest due to the properties such as increased hardness^1,2,3, toughness^4,5, and corrosion resistance^3,6. Recently, high-entropy oxides^7,8 and borides⁹ have been discovered, opening up a large playground for material enthusiasts. Oxides, in particular, can demonstrate useful and dynamic functional properties such as ferroelectricity¹⁰, magnetoelectricity^11,12, thermoelectricity¹³, and superconductivity¹⁴. Entropy-stabilized oxides (ESOs) have recently been shown to possess interesting, compositionally-dependent functional properties^15,16, despite the significant disorder, making this new class of materials particularly exciting.

Entropy-stabilized materials are chemically homogeneous, multicomponent (typically having five or more constituents), single-phase materials where the configurational entropic contribution () to the Gibbs free energy () is significant enough to drive the formation of a single phase solid solution¹⁷. The synthesis of multicomponent ESOs, where cationic configurational disorder is observed across the cation sites, requires precise control over the composition, temperature, deposition rate, quench rate, and quench temperature^7,16. This method seeks to enable the practitioner the ability to synthesize phase pure and chemically homogeneous entropy-stabilized oxide ceramic pellets and phase pure, single crystalline, flat thin films of the desired stoichiometry. Bulk materials can be synthesized with greater than 90% theoretical density enabling the study of the electronic, magnetic, and structural properties or use as sources for thin film physical vapor deposition (PVD) techniques. As the entropy-stabilized oxides considered here have five cations, thin film PVD techniques that employ five sources, such as molecular beam epitaxy (MBE) or co-sputtering, will be presented with the challenge of depositing chemically homogenous thin films due to flux drift. This protocol results in chemically homogenous, single crystalline, flat (root-mean-square (RMS) roughness of ~0.15 nm) entropy-stabilized oxide thin films from a single material source, which are shown to possess the nominal chemical composition. This thin film synthesis protocol may be enhanced by the inclusion of in situ electron or optical characterization techniques for real-time monitoring of the synthesis and refined quality control. Expected limitations of this method stem from laser energy drift which may limit the thickness of high quality films to be below 1 μm.

Despite the significant advances in the growth and characterization of thin film oxide materials^{10,18,19,20,21}, the correlation between stereochemistry and electronic structure in oxides can lead to significant differences in the final material stemming from seemingly insignificant methodological differences. Furthermore, the field of multicomponent entropy-stabilized oxides is rather nascent, with only two current reports of thin film synthesis in the literature^7,16. ESOs lend themselves particularly well to this process, circumventing challenges that would be presented by chemical vapor deposition and molecular beam epitaxy. Here, we provide a detailed synthesis protocol of bulk and thin films ESOs (Figure 1), in order to minimize materials processing difficulties, unintended property variations, and improve the acceleration of discovery in the field.

프로토콜

Caution: Wear necessary personal protective equipment (PPE) including close-toed shoes, full length pants, safety glasses, particulate filtration mask, lab coat, and gloves as oxide powders pose a risk for skin contact irritation and eye contact irritation. Consult all relevant material safety data sheets before beginning for additional PPE requirements. Synthesis should be done with the use of engineering controls such as a fume hood.

1. Bulk Synthesis of Entropy-stabilized Oxides

1. Mass Calculation of Constituent Oxide Powders
   1. Estimate the total desired mass of the target by multiplying the desired volume by the average density of the constituent binary oxides.
      
      
      where and are the mole fraction and the density of the th component. For a 1" (2.54 cm) diameter, ⅛" (0.3175 cm) thick sample, the target volume is 1.7 cm³.
   2. Determine the required moles of each component by dividing this target mass by the average molar mass of the constituent binary oxides.
      
      
      where is the molar mass of the th component. Convert the number of moles, , back to grams by
      
      NOTE: The masses of constituents and targeted compositions of the materials synthesized here are given in Tables 1 and 2.
2. Preprocessing of Oxide Powders
   1. Clean an agate pestle and mortar by etching with 20 mL of aqua regia (HNO₃ + 3 HCl). Pour the acid into the mortar and grind with the pestle until the bottom is clear. Dispose of the acid properly and rinse with water.
   2. Combine 0.559 g of MgO, 1.103 g of CoO, 1.035 g of NiO, 1.103 g of CuO, and 1.129 g of ZnO (for equimolar composition) powders in the clean mortar.
   3. Using the clean pestle, grind the powder using clockwise motions for 20 turns, then 20 counterclockwise turns. Repeat this process for at least 45 min. Use a clean metal spatula to remove powder from the sides of the mortar and brush the powder down to the center of the mortar.
      NOTE: Powder mixing and grinding are complete when the powder is homogenous and grey-black in color, appears finely ground, and feels smooth.
   4. Transfer the powder into a clean, sealable container for transport.
3. Ceramic Pellet Pressing
   CAUTION: Wear gloves and safety glasses when assembling the die and while the press is in use. Perform entire die cleaning and assembly steps on a clean paper surface. The components used are shown in Figure 2.
   1. Lubricate the sides and interior face of the small bottom plunger (labelled C in Figure 2a and 2b) of the die with mineral oil and insert into the die cylinder until it is flush with the bottom.
   2. Roll a weigh paper into the cavity of the die so that the sides of the die are covered. Pour the powder into the bottom of the die. Without allowing the small plunger to fall out of the die, gently tap the part on the counter to remove any air pockets and level the powder. Carefully remove the weigh paper.
   3. Add a small amount of acetone to the powder in the cavity of the die to form a slurry. This enables grain flow while the target is under pressure and inhibits the formation of voids.
   4. Lubricate the sides and interior face of the plunger (part B in Figure 2a and 2b) with paraffin oil, being careful to not disturb the powder. Insert this part into the die. Place the assembled die into the pressing machine as pictured in Figure 2c, including the top and bottom plates (parts D in Figure 2a and 2b) to provide an even surface.
   5. Place die in the cold uniaxial press. Pump the press arm until 200 MPa is reached. Allow the press to sit in the compressed state for 20 min. The pressure will relax with time as the powder densifies. Add pressure as needed to maintain 200 MPa for the duration of pressing. Wipe away any excess solvent that leaks out of the die.
   6. Release the press pressure. Carefully remove the top and bottom plates. Position the removal sheath and removal piston as shown in Figure 2c. Press slowly, removing the small die piece from the assembly before exposing the pressed target. Press the assembly carefully until the target is exposed from the die. Carefully remove the green body and transfer to a crucible for sintering.
4. Ceramic sintering
   CAUTION: Target materials will be quenched from high temperatures. Wear heat resistant gloves and a face shield when removing the crucible from the hot furnace.
   1. Obtain an alumina crucible that will fit the pressed powder and a 2 mm layer of Yttria-Stabilized Zirconia (YSZ) 0.1–0.2 mm beads. Coat the bottom of the crucible with YSZ beads.
      NOTE: The coating should be approximately 2 mm in thickness to ensure that the target does not contact the bottom of the crucible.
   2. Slowly and carefully transfer the pressed target to the center of the crucible.
   3. Using metal tongs, carefully transport the crucible to the sintering furnace. Increase the temperature to 1100 °C at 50 °C min^-1. Sinter the target for 24 h at 1,100 °C in an air atmosphere.
   4. While at 1100 °C, remove the crucible from the furnace. Using tongs, quickly quench the target in room temperature water. The target will sputter for ~30 s, then remove it from the water and set to dry.
   5. Once target is cool and dry, measure the target density and compare to the theoretical value, , calculated in Part 1. Measure the mass of the target on the balance used previously, and measure the dimensions using calipers. The ratio of the measured density to the estimated value, , gives the percent theoretical density.
      NOTE: After the synthesis, the density is usually ~80% of the theoretical density.
   6. For higher density, regrind the sintered target using the pestle and mortar and repeat the Bulk Synthesis procedure from step 1.2.3. After the second sintering, determine the density of the target.
      NOTE: Usually the measured density is theoretical density, which is suitable for pulsed laser deposition (PLD).

2. PLD of ESO Single Crystal Films

1. Target preparation
   1. The bulk ceramic pellets synthesized in step 1 will now serve as deposition sources (targets). Polish the target(s) in a circular motion using progressive (320/600/800/1,200) grits of SiC paper until the surface is reflective and uniform.
   2. Place the targets on the rotating carousel inside the chamber and place a ~2 cm x 2 cm piece of burn paper on the final target in the beam path.
   3. Measure the laser spot size by firing a single shot at the target and measuring the resulting burn mark across both axes. If the spot size is not correct, adjust the focusing lens (Figure 3a). Adjust the measured spot size until an ellipse, 0.27 cm x 0.24 cm across both axes is achieved.
   4. Remove the burn paper and close the door for evacuation. Evacuate the chamber using a dry scroll roughing pump to a pressure of 6.7 Pa, at which point the turbo pump can be spun up to a rate of 1,000 Hz.
   5. Pump out the chamber to a base pressure of at least 1.3 x 10^-5 Pa as measured by an ion gauge. Once reached, reduce the turbo to a speed of 200 Hz to allow the use of process gas during the growth.
2. Substrate Preparation
   1. Clean a single crystalline, one side polished, 0.5 mm thick MgO substrate by sonication for 2 min each in semiconductor grade trichloroethylene (TCE), semiconductor grade acetone, and high purity isopropanol (IPA).
   2. Blow the substrate off with ultra-dry, compressed N₂ gas, and attach the substrate to the substrate platen (Figure 3b) with a small amount of thermally conductive silver paint. Heat the substrate and platen to °C for 10 min on a hot plate to cure the silver paint.
   3. Using the external transfer tool, place the substrate holder on the transfer arm in the chamber load lock, then seal and pump out the chamber to a pressure of at least 1.3 x 10^-4 Pa.
   4. Transfer the substrate into the growth chamber by opening the gate valve between the two and using the transfer arm to place the substrate platen on the heater assembly.
   5. Retract the transfer arm back into the load lock and seal the gate. Lower the heater using the screw assembly on top of the chamber.
3. Laser Energy and Fluence
   NOTE: Deposition is enabled by the irradiation from a 248 nm KrF pulsed excimer laser. The laser pulse width is ~20 ns.
   1. Measure the laser energy using an energy meter placed in the beam path, just before entering the chamber (Figure 3a). Determine the mean energy after irradiating the photodiode with 50 pulses at a rate of 2 Hz.
   2. Vary the excitation voltage of the laser until an average pulse energy of 310 mJ is reached with ± 10 mJ stability. Remove the energy meter from the beam path to allow the laser to pass into the chamber.
      NOTE: Using a laser attenuation of the chamber window of 10%, the configuration above gives a fluence of 2.55 J cm^-2. The substrate-target distance in this work is 7 cm. A different substrate-target difference may change ideal deposition conditions and growth rate.
4. Deposition
   1. Before growth, heat the substrate to 1,000 °C for 30 min at a rate 30 °C min^-1 in vacuum to dehydroxylize the surface of MgO crystal. Reduce the temperature to 300 °C at 30° min^-1 and allow to equilibrate for 10 min.
      NOTE: Our reported temperatures are determined by a thermocouple within the heater block.
   2. Flow ultra-high purity (99.999%) O₂ gas into the chamber to reach a pressure of 6.7 Pa.
      NOTE: When oxygen flown into the chamber, the pressure is measured using a barotron gauge. The gas is introduced using a mass flow controller, as part of a closed loop system which stabilizes chamber pressure during growth.
   3. Clean the targets of any remaining contaminants and prepare them for growth by pre-ablation. Set the selected target to raster and rotate, so that the laser is not hitting the same spot each time, ensure that the substrate shutter is closed, and ablate the target for 2,000 pulses at a rate of 5 Hz.
      NOTE: The target is now prepared, and the system is at the correct conditions (temperature, pressure, fluence) for deposition.
   4. Open the shutter before deposition. At these conditions, 10,000 pulses at 6 Hz produces an ~80 nm thick film.
      NOTE: This growth rate was determined by X-ray reflectivity in previous work¹⁶.
   5. After deposition, increase the oxygen partial pressure to 133 Pa (1.0 torr) to inhibit the formation of oxygen vacancies. Reduce the sample temperature to 40 °C at 10 °min-1. Once 40 °C is reached, close the flow of oxygen and, after the stabilization of pressure, open the gate valve between the growth chamber and the load lock. Raise the heater and use the transfer arm to remove the substrate platen from the assembly back into the load lock.
   6. Vent the load lock to atmosphere and remove the sample using the external transfer tool. Remove the sample from the platen using a razor blade and polish the platen to take off the remaining silver paint and deposited material. Repeat the procedure starting from step 2.2 for additional film growth.

결과

X-ray diffraction (XRD) spectra were taken of both the prepared (Mg_0.25(1-x)Co_xNi_0.25(1-x)Cu_0.25(1-x)Zn_0.25(1-x))O (x = 0.20, 0.27, 0.33) and (Mg_0.25(1-x)Co_0.25(1-x)Ni_0.25(1-x)Cu_xZn_0.25(1-x))O (x = 0.11, 0.27) bulk ceramics (Figure 4a) and deposited thin films (Figure 4b). These data show that the samples are single phase...

토론

We have described and shown a protocol for the synthesis of bulk and high-quality, single crystalline films of (Mg_0.25(1-x)Co_xNi_0.25(1-x)Cu_0.25(1-x)Zn_0.25(1-x))O (x = 0.20, 0.27, 0.33) and (Mg_0.25(1-x)Co_0.25(1-x)Ni_0.25(1-x)Cu_xZn_0.25(1-x))O (x = 0.11, 0.27) entropy-stabilized oxides. We expect these synthesis techniques to be applicable to a wide range of entropy-stabilized oxide compositions as more are discovere...

자료

Name	Company	Catalog Number	Comments
MAGNESIUM OXIDE 99.95%	Fisher	AA1468422
COBALT(II) OXIDE, 99.995%	Fisher	AA4435414
NICKEL(II) OXIDE 99.998%	Fisher	AA1081914
COPPER(II) OXIDE 99.995%	Fisher	AA1070014
ZINC OXIDE 99.99%	Fisher	AA8781230
TRICHLROETHLENE SEMICNDTR 9	Fisher	AA39744K7
ACETONE SEMICNDTR GRD 99.5%	Fisher	AA19392K7
2-PROPANOL ACS 99.5%	Fisher	A416S4
Mineral oil, pure	Acros Organics	AC415080010
alumina crucible	MTI Corporation	eq-ca-l50w40h20
ZIRCONIA (YSZ) GRINDING MEDIA	Inframat Advanced Materials	4039GM-S010
SiC paper 320/600/800/1200	South Bay Technology	SDA08032-25
MgO (100) substrate, 5x5x0.5 mm, 1SP	MTI Corporation	MGa050505S1
OXYGEN COMPRESSED ULTRA HIGH PURITY GRADE, 99.999%	Cryogenic Gases	OXYUHP
NITROGEN COMPRESSED EXTRA DRY GRADE	Cryogenic Gases	NITEX