Simple alkyllithium compounds with low mass radicals (Me, Et) have low volatility, and seem to have polymeric structure even the gaseous state, according to mass spectra and IR spectrometry [[i],[ii]] They are therefore not having high potential in being used as precursor for MOCVD growth of lithium-containing layers.
Some longer (or branched) radicals alkyl- or arylithium compounds were reported to be monomeric in THF solvent: tert-butyllithium LitBu, , supermesityllithium, mesityllithium, and even phenyllithium LiPh and sec-butyllithium LisecBu have monomer/dimer equilivria in THF solution,
although in hydrocarbon solvents and in diethyl ether they have higher nuclearity (tetramers or dimers) [[iii]]
[i] J. Berkowitz, D.A. Bafus, T.L..Brown, J.Phys.Chem., 1961, 65, p.1380
[ii] R. West, W. Glaze, J. Am. Chem. Soc., 1961, 83, p.3580
[iii] W. Bauer, W.R. Winchester, P. von R. Schleyer, Organometallics, 1987, 6 (11), pp.2371; DOI: 10.1021/om00154a017, http://pubs.acs.org/doi/abs/10.1021/om00154a017, « Monomeric organolithium compounds in tetrahydrofuran: tert-butyllithium, sec-butyllithium, supermesityllithium, mesityllithium, and phenyllithium. Carbon-lithium coupling constants and the nature of carbon-lithium bonding »
Lithium n-butyl (n-Butyllithium) LinBu was found to be hexameric in bezene and cyclohexane
[[i]]
[i]D. Margerison, J. P. Newport , Trans. Faraday Soc., 1963,59, 2058-2063, DOI: 10.1039/TF9635902058, Degree of association of n-butyl lithium in hydrocarbon media", http://pubs.rsc.org/EN/content/articlelanding/ 1963/tf/tf9635902058#!divAbstract
n-butyllithium LinBu has been studied as precursor for the ALD growth of lithium-containing layers. [30]
Sec-butyllithium LisecBu was tested as precursor for the MOCVD growth of Li-containing
materials like LiCoO2. However, with this precursor insufficient lithium was transported to the reactor zone, thus the MOCVD tests with LisecBu as lithium source were
abandoned, as desired materials could be easily obtained with air stable Li(thd) complex.[[i]]
[i] P Fragnaud, R Nagarajan, DM Schleich, D Vujic, J. Power Sources, 1995, Vol.54, Iss. 2, p.362–366, « Thin-film cathodes for secondary lithium batteries »
Tert-Butyllithium LitBu is relatively volatile compound. It has vapor pressure 0.1mm at 25°C. (as measured by by exposing tert-butyllithium vapor to a Hastings DV-43 vacuum gauge. [[i] ] LitBu was reported to decompose at 140°C.
The physical properties and structure of t-butyllithium were studied in [[ii]]
t-butyllithium is a tetramer or hexamer in hydrocarbon solution, with fluxional behaviour observed especially for hexamer having empty sites [[iii] ]
[i] JB Smart, R Hogan, PA Scherr, L Ferrier… - Journal of the …, 1972 - ACS Publications Calculated and observed electronic transitions in organolithium aggregates
[ii] M. Weiner, G. Vogel, R. West, Inorg. Chem., 1962, 1 (3), pp.654, DOI: 10.1021/ic50003a040, http://pubs.acs.org/doi/abs/10.1021/ic50003a040
[iii]R. D. Thomas , M.T. Clarke , R. M. Jensen , T. C. Young, Organomet., 1986, 5 (9), pp 1851, DOI: 10.1021/om00140a016, http://pubs.acs.org/doi/abs/10.1021/om00140a016 ,” Fluxional exchange of tert-butyllithium tetramers from temperature-dependent carbon-13-lithium-6 coupling”.
Tert-butyllithium LitBu, combined with cobalt dicarbonyl cyclopentadienyl CoCp(CO)2, was applied for the growth of
LiCoO2 thin films by low pressure CVD, for thir application as cathodes for all solid-state microbatteries. The influence of the deposition parameters (deposition temperature, concentration of the various reactants, duration of thin-film growth) on the properties
of LPCVD grown thin-film LiCoO2 cathodes was systematically investigated. The LiCoO2 layers were studied by XRD, inductively coupled plasma–atomic emission spectrometry,
RBS and electrochemical analyses. Stoichiometry of the films was controlled by varying the precursor flows. High crystallinity and a high electrochemical activity was obtained for the samples deposited at high temperatures having optimum stoichiometry, with
a storage capacity corresponding to a reversible Li-content around the theoretical value of 0.55 per Co. [[i]]
[i] J. F.M. Oudenhoven, T. van Dongen, R. A.H. Niessen, M. H.J.M. de Croon, P. H.L. Notten
http://jes.ecsdl.org/content/156/5/D169.short , « Low-Pressure Chemical Vapor Deposition of LiCoO2 Thin Films: A Systematic Investigation of the Deposition Parameters »
Tertiary-butyllithium LitBu was reported to be tested as Li source (p-dopant) for the deposition of Li-doped ZnSe on Gaas substrates by MOCVD . However, the success was reported
to be only marginal. Li concentrations in ZnSe layers up to 4×1017 cm-3 were achieved by optimising of the LitBu carrier
gas flow rate. Increase of LitBu carrier gas flow,resulted in increase of acceptor concentration and in increase of carrier concentrations NA − ND to ~1×1015 cm-3, according to 4.2 K photoluminescence and C − V measurements, respectively. [[i]] Another report about p-doping of ZnSe using LitBu was mentioned in [[ii]]
[i] H. Mitsuhashi, A. Yahata, T. Uemoto, A. Kamata, M. Okajima, K. Hirahara and T. Beppu, Journal of Crystal Growth, 1990, Vol.101, Iss. 1-4, pp. 818-821, "p-type carrier concentration control in lithium-doped zinc selenide grown by MOCVD"
[ii] Shizuo Fujita and Shigeo Fujita, SEMICONDUCTORS AND SEMIMETALS, VOL. 44, CHAPTER 2, Growth and Characterization of ZnSe-based 11-VI Semiconductors by MOVPE
Chapter 2 Growth and Characterization of ZnSe-based II-VI Semiconductors by MOVPE, and refs therein: (Yahata et al., 1990)
Tertiary-butyl-lithium (tBLi) combined with ethyl-iodide EtI was used for the growth of Li,I co-doped ZnSe films by atmospheric-pressure AP MOVPE (with ZnEt2 and SetBuiPr as
Zn and Se sources) at 450°C. Li has the well-known metastability of substitutional acceptor sites and interstitial donor sites, therefore the examination of the stability of Li sites was studied by using iodine co-doping. Photomuninescence (PL) spectra
at 13 K of only Li-doped ZnSe films were dominated by neutral-donor-bound excitons, whereas PL spectra of Li,I-co doped ZnSe layers demonstrated systematic shift to the neutral-acceptor-bound excitons by increasing co-doping with iodine. The electrical properties
correspondingly turned-over from n- to p-type [[i]]
[i] Suemune, H. Ohsawa, T. Tawara, H. Machida and N. Shimoyama, J. Cryst. Growth, 2000, Vol. 214-215, pp.562-566, “Study of site change of Li impurities in ZnSe by co-doping with iodine”
Tertiarybutyllithium LitBu was applied as an acceptor dopant source (with ZnMe2 and SeMe2 as main element sources) for the growth of Li-doped p-type ZnS epitaxial layers
on (001)-oriented GaAs substrates by low-pressure MOCVD. As-grown samples demonstrated electrically high resistive n-type or p-type conductivity; however, after annealing in N2 atmosphere at 550°C all samples exhibited p-type conductivity, with the highest
net acceptor concentration (NA−ND) reaching ca. 4.1×1016 cm−3. Lithium-doped ZnS epitaxial layers NA−ND<1015 cm−3 were dominated by the radiative recombination of excitons bound to a lithium acceptor at 327.6 nm, as was found
from PL spectra. On the other hand, PL spectra of lithium-doped ZnS epitaxial layers with NA−ND>1015 cm−3 were dominated by free-to-acceptor emission with LO phonon replicas. From the peak energy of the FA emission, the lithium acceptor ionization
energy was estimated to be about 196 meV.[[i]]
[i] S. Nakamura, J. Yamaguchi, Shinsuke Takagimoto, Yoichi Yamada, Tsunemasa Taguchi, Journal of Crystal Growth, Volumes 237-239, Part 2, April 2002, Pages 1570-1574, Luminescence properties of lithium-doped ZnS epitaxial layers grown by MOCVD