Construction and application of a novel combination glove box deposition system to the study of air-sensitive materials by tunneling spectroscopy

Cite as: Review of Scientific Instruments 55, 1120 (1984); https://doi.org/10.1063/1.1137895 Submitted: 27 February 1984 . Accepted: 20 March 1984 . Published Online: 04 June 1998

K. W. Hipps, and Ursula Mazur

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© 1984 American Institute of Physics. Construction and application of a novel combination glove box deposition system to the study of air-sensitive materials by tunneling spectroscopy K. W. Hipps and Ursula Mazur Department of Chemistry and Chemical Physics Program, Washington State University, Pullman, Washington 99164-4630 (Received 27 February 1984; accepted for publication 20 March 1984)

The construction and application of a high-vacuum deposition system housed in a recirculating, catalytically scrubbed, inert-atmosphere glove box is reported. This system is specifically applied to the fabrication of tunnel diodes used in a surface vibrational spectroscopy called inelastic electron tunneling spectroscopy or lETS. Through the use of this inert-atmosphere adsorption/ fabrication system, tunneling spectra have been obtained from a variety of air-sensitive compounds adsorbed on aluminum oxide. Up to now, spectra of some of the species reported here have been unattainable by the adsorption techniques used in lETS. The test molecules employed in this study included TCNE (tetracyancethylene), TCNQ (tetracyanoquinodimethane), and

CO2(CO)s' TCNE adsorbed reactively on thin-film alumina under nitrogen to form a species with a vibrational spectrum similar to that of the TCNE-- 2 ion, while TCNQ appears to form the monoanion under the same adsorption conditions.

INTRODUCTION I. EXPERIMENTAL SETUP Inelastic electron tunneling spectroscopy (lETS), or tunnel­ A. Glove box deposition system ing spectroscopy, is a successful surface technique used to measure the molecular vibrations, 1-3 and in some cases elec­ The glove box unit (model HE-453-2 Dri-Lab) was pur­ tronic transitions,4 of a wide range of organic,5-s inorgan­ chased from Vacuum Atmospheres Co. The approximate ic, 9-11 and biological compounds 12.13 absorbed on an insulat­ dimensions of the box are 115 X 33 X 33 in. (the length of the ing substrate forming the barrier of a metal-insulator-metal air lock is included in the lIS-in. total length measurement). tunnel junction. Although the junction fabrication requires a This unit has three work stations with viewing panels and high-vacuum system, the adsorption step itself is often per­ glove ports. The fourth station (at the air-lock end) is covered formed outside the vacuum chamber. Here, the molecule of with an aluminum panel. Ten lIS-Vat 20-A, one 208-V at interest is applied as a neat liquid or from solution onto a 30-A, and two 20-V at 4OO-A power input receptacles were substrate and the excess is spun off. Alternatively, volatile provided by the manufacturer. Also provided were two iso­ and sublimable absorbents can be introduced directly into lated ground BNC connectors, a 50-pin feedthrough which the vacuum system. Whereas the first adsorption method is presently used for the quadrupole control lines, and two allows one to study only those molecules which are stable in fluid inlet ports. The atmosphere in the box is constantly air, the second method permits working with air-sensitive recirculated at the rate of 40 cfm through a catalyst bed materials provided they can be vaporized. Thus, air-sensitive (model MO-40-IV Dri-Lab) located in a separate unit sta­ materials which are not stable in the gas phase could not be tioned outside the glove box. The catalyst removes oxygen to studied by lETS. - 1 ppm and water vapor to S 10 ppm. Only ultrapure-qual­ Until now tunneling spectroscopists have been limited ity gases are used in the box. A deposition system of our own to studying those molecules which can be adsorbed under design and fabricated in the University machine shop is the conditions described above. In this paper we report on housed in the work station opposite the air lock (see Fig. 1). the construction and the application of a high-vacuum depo­ The deposition system is composed of a stainless-steel sition system housed in a recirculating, catalytically tooled collar base (l2X 6 in. high) and a glass bell jar (12X 10 scrubbed, controlled-atmosphere glove box. We have suc­ in.). The system is pumped by a 4-in. oil-diffusion pump with cessfully utilized this novel setup for preparing tunnel junc­ an integral liquid-nitrogen trap. A pressure of 2 X 10- H Torr tions from which tunneling spectra of air- and water-sensi­ can be obtained in 1 h. The stainless-steel collar base has tive compounds, adsorbed on alumina from solution, can be seven side ports fitted with three high-current feedthroughs, obtained. The molecules studied via our controlled atmo­ a thermocouple gauge, the head of a quadrupole residual gas sphere system include TCNE, TCNE - 1,14 TCNE - 2,14 analyzer (model SX-200 from VG. Co.), and a precision leak

TCNQ, and CO2(CO)8' The results of part of these studies, valve. Rotary motion, glow discharge, octal electrical, and and a comparison with results obtained with conventional gas inlet feedthroughs are mounted on the bottom of the tunneling techniques, are reported here. tooled collar base. The interior of the collar base is fitted with

1120 Rev. Sci. Instrum. 55 (7), July 1984 0034-6748/84/071120-05$01.30 © 1984 American Institute of Physics 1120 ether and acetone can be handled successfully in this man­ ner. Typically, one keeps the doping chamber under a slight vacuum when injecting the solution containing the adsor­ bate through the septum. This prevents any solvent conta­ mination of the atmosphere in the glove box, and holds the bell jar in place during the doping procedure. Once the sub­ strate is exposed to the desired compound, the excess solu­ tion is spun off and the vacuum chamber IS evacuated to about 300 mTorr. The doping chamber is then partially e back-filled with nitrogen. The evacuationlback-fill proce­ dure is repeated three or more times in order to remove the residual solvent vapor in the chamber. At this point the dop­ ing chamber is opened and the substrate is returned to the vacuum system for top-metal deposition. FIG. I. Schematic representation of the glove box deposition system: (a) de­ Solution preparation, handling, and storage are essen­ position chamber, (b) doping chamber, (c) spinner, (d) air lock, (e) to high tial features to realizing the potential of this fabrication sys­ vacuum, (t) to roughing pump. tem and to prevent contamination of the system. We are presently using the Kontes Scientific line of microflex vials and valves as preparation/storage vessels. These valves are removable metal shields which prevent evaporated materials all Teflon in construction and are also equipped with re­ from coating the walls of the deposition system. The configu­ placeable silicon rubber septa. Aldrich Chemical offers a ration of the deposition sources and the shields allow for easy similar line which we are planning to try. Sample prepara­ handling and removal. Thus, they can be cleaned without tion is as follows: The solute is added to the vial (under inert compromising the integrity of the glovebox atmosphere. conditions if necessary) and a valve with a fresh septum is The quadrupole mass spectrometer serves multiple used to seal the vial. A 22-gauge needle is used to puncture duty in this system. It can be used to monitor the residual the septum and provide entry to the vial. The needle is con­ gases in the vacuum chamber, or it can be used to follow the nected to a manifold having high-purity N2 gas and vacuum desorption of adsorbed species. Further, by utilizing the con­ available. The vial is alternately evacuated and back-filled trolled leak valve mounted on the collar base, the entire de­ with N2 gas. The freshly distilled deairated (and if necessary, position system becomes a mass spectrometer for character­ dried) solvent is transferred by syringe to the vial and further izing the content and quality of the atmosphere in the glove pump/back-fill cycles are performed. The Teflon valve is box. In this latter context, it is principally used to check for closed and the sample is transported through the air lock of contamination by organic molecules and water. The atmo­ the glove box/deposition system. sphere leak integrity of the glove box is determined external­ We found that junction fabrication can be performed in ly by a Leybold Utratest M2 "sniffer-type" helium leak de­ this glove box deposition system with relative ease and that tector. This leak-test procedure is repeated at monthly the atmosphere in the glove box is sufficiently hydrocarbon intervals to ensure the integrity of the gloves. A 60-W tung­ free. Blank alumina substrates can be exposed to the glove sten filament bulb having a large opening in its envelope is box atmosphere for periods of 1/2 h or longer and their tun­ allowed to bum in the glove box at all times. It serves as a neling spectra show no signs of hydrocarbon contamination. catastrophe indicator and the mean time to failure for one of these bulbs is about 3 weeks. B.IETS In order to prevent glove box atmosphere contamina­ tion and catalyst poisoning by solvents used during the ad­ Tunneling spectra were obtained with a spectrometer sorption step, we have installed a special doping chamber in described previously. IO The junctions reported in this study the box. This chamber consists of a spinning motor mounted are AI-AI20 3-Pb structures. Because of the small amount of on a 10-in. pump plate and a lOX 71/2-in. polycarbonate bell water vapor present in the glove box deposition system, alu­ jar with an off-center septum port on top. An aluminum foil mina insulators suitable for doping had to be grown in a 100- lining is used to prevent direct contact between organic li­ mTorr 300-V ac-driven oxygen plasma for a period of 20 quids and the walls of the jar. This vacuum chamber is evac­ min. In a conventional deposition system surrounded by uated by an external roughing pump and brought up to pres­ room air, alumina substrates suitable for adsorption studies sure with the gas atmosphere in the glove box through a by tunneling spectroscopy can be grown in 3 min under the two-way stopcock attached to the pump plate. The doping same conditions. solution is transferred with a glass syringe equipped with a Compounds were adsorbed from acetone and benzene two-way stainless-steel stopcock (Luer joints) from a storage solutions at concentrations ranging from 0.01 up to 1 mg/ vessel (vide infra) into the doping chamber through the sep­ ml. All solutions were prepared and stored under nitrogen. tum on the polycarbonatejar. We found that this procedure Compounds were doped both under inert and room air con­ can be performed easily and with minimal contamination of ditions. TCNE was also evaporated onto the aluminum ox­ the glove box atmosphere. Even very volatile solvents like ide substrate from a quartz tube connected to the main vacu-

1121 Rev. Sci. Instrum., Vol. 55, No.7, July 1984 Tunneling spectroscopy 1121 urn chamber by a valve. The tube was heated to 30°C, the identified as that oftricyanovinylalcoholate anion (TV A). IS valve was opened, and the oxide was exposed to the TCNE The spectrum of TCNE adsorbed under inert conditions is vapor for a period of 5 min. quite complex and is partially discussed elsewhere. 14 All spectra were recorded at 4.2 K with modulation Figure 2(c) presents the tunneling results obtained from voltages below 2 m V and signal levels of about 500 n V. Most vapor phase adsorption of TCNE on thin-film alumina. are low-resolution spectra. Aside from the solvent peaks present in spectrum (b), Figs. 2(b) and 2(c) are quite similar. The results presented in Fig. 2 indicate that TCNE adsorbed from solution under inert con­ c. Materials ditions produces the same surface species as TCNE deposit­ TCNE (tetracyanoethylene), TCNQ (tetracyanoquino­ ed on alumina in the absence of solvent and under high­ dimethane), and Coz(CO)s were obtained from Aldrich vacuum conditions. Clearly, these are not the same species as Chemical Company and from Alfa Products. TCNE was is present when conventional solution adsorption is used sublimed twice over charcoal before use while COz(CO)s and [Fig. 2(a)). TCNQ were used without further purification. All com­ Figure 3 presents the lET spectrum of Coz(CO)s ad­ pounds were stored under nitrogen. All solvents used were sorbed from benzene solution on alumina under inert condi­ reagent quality and freshly distilled under flowing N z and tions. This is a very intense spectrum as evidenced by the from over molecular sieves just before use. strong, low-frequency metal-ligand motions. When the Coz(CO)s adsorption procedure is performed in air the resultant tunneling data show no indication of any II. EXPERIMENTAL RESULTS type of surface complex. The lET spectra are those of blank Figure 2 contrasts the tunneling spectra obtained fol­ junctions. This result is due to both the relative instability of lowing adsorption of TCNE from benzene solution under the complex in air, and the well-known reactive decomposi­ atmospheric (a) and inert (b) conditions. The difference in the tion reaction between a metal oxide and a metal carbonyl in spectral results is certainly dramatic. This is particularly em­ the presence of oxygen in which the complex loses its CO's phasized by the number and the frequencies of the CN and forms the corresponding metal oxide. 16 stretches observed in the two tunneling spectra. Spectrum (a) The dicobalt octacarbonyl species has several struc­ exhibits only one strong CN band at 2214 cm - I. Spectrum tures depending on its environment. In the solid state,

(b) has three peaks in the CN stretching region located at Coz(CO)s has one structure of CZv symmetry with two bridg­ I 2064,2160, and 2251 cm- • The tunneling spectrum ob­ ing CO'S.17.IS In solution, it can exist in up to three different tained from TCNE adsorbed on alumina, in air, has been forms depending on the solvent and temperature. 17.IS Table I compares the vibrational frequencies of the different Coz(CO)s structures with the vibrational modes observed in the tunneling spectrum in Figs. 3. The frequencies of the CO

'"OJ CoZ(CO)S/benzene

>- I- H Ul If) Z )- If) w I- f-- H (D Z (D H (f] '" If) Z W ~~ I- "If)'" Z H (

IZI 121Z11Z1 241Z11Z1 ENERGY (cm-1)

FIG. 2. Tunneling spectra of surface species resulting from adsorption of TCNE under different environmental conditions. (a) TCNE adsorbed from 121 12121121 24121121 benzene in air. This spectrum was recorded at 4.2 K with 1.8-mV rms mo­ ENERGY (=rn- 1) dulation. (b) TCNE adsorbed from benzene under inert conditions. This spectrum was measured at 4.2 K with 1.8-mV rms modulation. The bands FIG. 3. The tunneling spectrum of dicobalt octacarbonyl adsorbed on alu­ marked • are solvent bands. (c) TCNE adsorbed by vapor deposition under mina from benzene solution under a nitrogen atmosphere. This spectrum high-vacuum conditions. was measured at 4.2 K with 1.8-mV rms modulation.

1122 1122 Rev. Sci.lnstrum., Vol. 55, No.7, July 1984 Tunneling spectroscopy TABLE I. Tunneling and IR frequencies (cm - ') for Co2(CO) •. lET IR

From benzene Solutionb a b solution Solid • (Bridged) (D3d)

2112 2107 2075 2071 2069 >­ f- 2064 2059 H 2044 2042 (f) Z 2035 2031 2031 W 2028 2023 f­ 2001 1991 Z 1910rn 1886 1867 1859 1857 1735w 626 570sh 579 550 sh 515s 522 515 sh 529 121 12121121 24121121 508 ENERGY (om-l) 435sh 482 483 465 FIG. 4. Tunneling spectra obtained from junctions doped with TCNQ in 395 sh acetone solutions; (a) O.Ol-mglml solution adsorbed under inert conditions, 381 384 388s (b) 0.06-mglml solution adsorbed in air, and (c) 0.2-mglml solution ad­ sorbed in air. All spectra were recorded at 4.2 K. Solvent bands are indicat­ "Reference 17. ed (*). b Reference 18.

atmosphere fabrication apparatus with those obtained by stretching modes in the lET spectrum are broad and down­ conventional means. This is because the surface species shifted by about 100 cm -I. To understand this, it should be which results does not depend on adsorption conditions. recalled that the top-metal electrode used in tunneling spec­ Coleman23 has previously reported the spectrum of troscopy has the effect of downshifting the frequencies and TCNQ adsorbed on alumina under atmospheric conditions. broadening of the vibrational modes,19 an effect which is He has identified the surface product as the monoanion radi­ generally quite insignificant except for chemisorbed CO and cal of TCNQ. Figure 4 clearly indicates that the species OH.20.21 formed under inert conditions is the same. There are differ­ We assign the 191 O-cm - 1 mode in the tunneling spec­ ences, however, between the atmospheric and inert adsorp­ trum to the terminal CO ligands, and the broad feature near tion spectra. These differences are worth noting since they 1730 cm -I to bridging CO ligands. These assignments agree have also occurred in many other spectra obtained by us. well with the tunneling data of CO chemisorbed on metal First, the adsorbate concentration required for 'good" spec­ particles.3.19-.22 In the case of CO on alumina supported rho­ tra is considerably less in the case of glove box adsorption. dium, the high-frequency bands were assigned to the CO Note that the 0.01 mg/ml solution [Fig. 4(a)] gave a much stretching vibration of adsorbed bridged (1721 cm -\051and stronger spectrum when used in the glove box than did the linear or terminal (1921 cm - I) CO species. 20 When highly 0.06-mg/ml solution used in air [Fig. 4(b)]. Second, the ad­ dispersed cobalt on alumina was exposed to saturation cov­ sorption of solvents, as signaled by the peaks marked with an erage of carbon monoxide only one high-frequency CO *, is considerably more pronounced in the case of junctions stretch due to a linear surface species was observed.3 The prepared in our inert atmosphere system. We believe that low-frequency metal-ligand bands at 390 and 511 cm -I seen both of these observations are due to a single characteristic of in Ref. 3 are also present in Fig. 3. The exact assignment of the oxide surface formed and utilized in our inert atmo­ the low-frequency modes requires isotopic substitution data sphere fabrication apparatus. Namely, that they are not cov­ and higher resolution spectra. However, on the basis of com­ ered by a heavy layer of surface water. parison with the IR frequencies and the previous tunneling results we can conclude that as a surface species CO (CO)s 2 III. DISCUSSION (adsorbed from solution) produces a structure that involves both terminal and bridging CO ligands. We have demonstrated that performing lETS studies in Figure 4 presents tunneling spectra obtained from the glove box deposition system described offers many ad­ TCNQ in acetone solutions under various concentration and vantages over conventional methods. The most obvious is adsorption conditions. Figure 4(a) is the spectrum obtained that one can study those compounds which are air sensitive when a O.OI-mg/ml solution is used for adsorption under and have no significant vapor pressure. Even in those cases inert conditions. Figures 4(b) and 4(c) are obtained when the where air-sensitive compounds can be vapor deposited, this adsorption takes place in air and when 0.06- and 0.2-mg/ml system allows secure handling of samples and far fewer ac­ solutions are, respectively, used. This set of spectra provide tive components in the residual gases present in the vacuum us with the ability to contrast results obtained in our inert chamber.

1123 Rev. Scl.lnstrum., Vol. 55, No.7, July 1984 Tunneling spectroscopy 1123 The test molecules that we have chosen provided some ACKNOWLEDGMENTS interesting insights into the adsorption processes which can This work was supported by the National Science occur under different environmental conditions. We found Foundation Solid State Chemistry Grant No. DMR- that TCNE adsorbed reactively on alumina both in air and in 8115978. The Foundation's continued support is appreciat­ nitrogen. Under atmospheric conditions the parent com­ ed. We also thank Professor M. P. Cooke for his assistance in pound formed the tricyanovinylalcoholate anion bound to a the synthetic aspect of this study and Douglas Benson for his surface aluminum ion; whereas under inert conditions, the help in fabricating the deposition system. surface species formed upon adsorption of TCNE is similar to TCNE - 2.14 Other surface reaction products may also be present. Although TCNQ adsorbs reactively on alumina, we found that the surface species formed did not depend on the presence of air during the adsorption step. This is in sharp contrast to what is observed in the case ofTCNE adsorption. Ip. K. Hansma, Phys. Rev. C 30,145 (1977). Dicobalt octacarbonyl could only be adsorbed under nitro­ 2W. H. Weinberg, Annu. Rev. Phys. Chern. 29, 115 (1978). gen due to its instability in air as well as its reactivity with the 3Tunneling Spectroscopy, edited by P. K. Hansma (Plenum, New York, substrate in the presence of oxygen. Good intensity tunnel­ 1982). ing results were obtained. 's. De Cheveigne, J. Klein, A. Leger, M. Belin, and D. Deffourneau, Phys. Although the construction of a glove box deposition Rev. B 15, 750 (1977). 's. D. Williams and K. W. Hipps, J. Catal. 78, 96 (1982). system is a costly venture, it opens new avenues of research 6U. Mazur and K. W. Hipps, J. Phys. Chern. 86, 2854 (1982). for tunneling spectroscopists. The most important of these is 7N. M. D. Brown,R.J. Turner,andD. G. Walmsley,J. Mol. Struct. 79,163 in the area of catalysis. There is great interest, for example, in (1982). 8 Vibrations at Sufaces, edited by D. R. Brund1e and H. Morawitz (Elsevier, identifying the molecular structure and catalytic reactivity Amsterdam, 1983). of homogeneous cluster compounds supported on metal ox­ "W. H. Weinberg, W. M. Bowser, and H. E. Evans, Surf. Sci. 106,489 ide surfaces. Many of these complexes, however, cannot be (1981). studied in air. Another area of interest lies in the study of the lOU. Mazur and K. W. Hipps, J. Phys. Chern. 85, 2244 (1981). 11K. W. Hipps and U. Mazur, Inorg. Chern. 20, 1391 (1981). metal oxides themselves in the context of their redox proper­ 12p. K. Hansma and R. V. Coleman, Science 184,1369 (1974). ties. Compounds used as indicators of surface redox activity l3N. M. D. Brown, R. B. Floyd, and D. G. Walmsley, J. Chern. Soc. Fara- generally form radicals which are extremely sensitive to at­ day Trans. 2 75, 261 (1979). mospheric conditions. Therefore, it is necessary to control l4U. Mazur and K. W. Hipps, J. Phys. Chern. 88, 1555 (1984). 15K. W. Hipps and U. Mazur, J. Phys. Chern. 86, 5105 (1982). the hydration and oxidation state of activated surfaces dur­ 16J. B. Kinney, R. H. Stanley, C. L. Reichel, and M. S. Wrighton, J. Am. ing the absorption process. These kinds of experiments re­ Chern. Soc. 103,4273 (1981). quire a glove box deposition system. Still another area not 17K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordina­ tion Compounds, 3rd ed. (Wiley, New York, 1978). much explored by tunneling spectroscopists is the construc­ I"D. M. Adams, Metal-Ligand and Related Vibrations (Edward Arnold, tion and characterization of new adsorbents. These types of London, 1967). studies might involve total synthesis of new substrates or 19J. R. Kirkley and P. K. Hansma, Phys. Rev. B 3, 2910 (1976). only require performing chemical modification of metal ox­ 2°R. M. Kroeker, W. C. Kaska, and P. K. Hansma, J. Catal. 57, 2 (1979). 21J. Klein, A. Leger, S. De Cheveigne, C. Guinet, M. Belin, and D. Defour- ides. In either case, it would be useful to conduct such experi­ neau, Surf. Sci. 82, L228 (1979). ments external to the vacuum deposition system under con­ nW. M. Dowser and W. H. Weinberg, J. Am. Chern. Soc. 102,4720(1980). trolled atmospheric conditions. 23C. S. Korman and R. V. Coleman, Phys. Rev. B 15,1877 (1977).

1124 Rev. Sci.lnstrum., Vol. 55, No.7, July 1984 Tunneling spectroscopy 1124