Cyclic Silylselenides: Convenient Selenium Precursors for Atomic Layer Deposition

Article abstract of DOI:10.1002/cplu.202000108

Three cyclic silylselenides were prepared in a straightforward manner. Property tuning has been achieved by varying the ring size and the number of embedded selenium atoms. All silylselenides possess improved resistance towards moisture and oxidation as well as high thermal robustness and sufficient volatility with almost zero residues. The six-membered diselenide proved to be particularly superior Se precursors for atomic layer deposition and allowed facile preparation of MoSe₂ layers. Their structure and composition have been investigated by Raman and X-ray photoelectron spectroscopy as well as scanning electron microscopy revealing vertically aligned flaky shaped nanosheets.

Full text of DOI:10.1002/cplu.202000108

Accepted Article
Title: Cyclic Silylselenides: Convenient Selenium Precursors for Atomic
Layer Deposition
Authors: Jaroslav Charvot, Daniel Pokorný, Raul Zazpe, Richard
Krumpolec, David Pavliňák, Luděk Hromádko, Jan Přikryl,
Jhonatan Rodriguez-Pereira, Milan Klikar, Veronika
Jelínková, Jan M. Macak, and Filip Bures
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content of this Accepted Article.
To be cited as: ChemPlusChem 10.1002/cplu.202000108
Link to VoR: http://dx.doi.org/10.1002/cplu.202000108
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Cyclic Silylselenides: Convenient Selenium Precursors for
Atomic Layer Deposition
Jaroslav Charvot,[a] Daniel Pokorný,[a] Raul Zazpe,[b,c] Richard Krumpolec,[d] David Pavliňák,[d] Luděk
Hromádko,[b,c] Jan Přikryl,[b] Jhonatan Rodriguez-Pereira,[b] Milan Klikar,[a] Veronika Jelínková,[e] Jan M.
Macak,[b,c] and Filip Bureš*[a]
[a]
J. Charvot, D. Pokorný, Dr. M. Klikar, Prof. F. Bureš
Institute of Organic Chemistry and Technology
Faculty of Chemical Technology, University of Pardubice
Studentská 573, Pardubice, Czech Republic
E-mail: filip.bures@upce.cz
[b]
[c]
[d]
[e]
Dr. R. Zazpe, L. Hromádko, J. Přikryl, J. Rodriguez-Pereira, Dr. J. M. Macak
Center of Materials and Nanotechnologies
Faculty of Chemical Technology, University of Pardubice
Nám. Čs. Legií 565, 53002 Pardubice, Czech Republic
Dr. R. Zazpe, L. Hromádko, Dr. J. M. Macak
Central European Institute of Technology
Brno University of Technology
Purkyňova 123, 61200, Brno, Czech Republic
Dr. R. Krumpolec, Dr. D. Pavliňák
R & D Center for Low-Cost Plasma and Nanotechnology Surface Modifications
Department of Physical Electronics, Faculty of Science,
Kotlářská 267/2, 611 37 Brno, Czech Republic
V. Jelínková
The Institute of Technology and Business in České Budějovice
Okružní 517/10, 370 01, České Budějovice, Czech Republic
Supporting information for this article is given via a link at the end of the document.
Abstract: Three cyclic silylselenides were prepared in
a
photocatalytic activity; MoSe2 is a very promising candidate in this
respect. It turned out to be very efficient photodegradation
catalyst[6,7] that may also replace Pt-based catalysts used in
electrochemically-assisted hydrogen evolution techniques.[8,9]
The surface area may be further increased by ALD on nanotubes
or nanoparticles, which generally lead to pronounced
photocatalytic properties. However, the current portfolio of
available selenium precursors for ALD is very limited. The most
simple precursor is elemental selenium,[10,11] which has
unfortunately very low volatility. A direct use of selenium is limited
to preparation of simple selenides such as ZnSe or CdSe. On the
contrary, very volatile H2Se represents versatile Se precursor
even for larger surfaces, [12] but its wider application is confined by
its high toxicity. Diethyl(di)selenide Et2Se (Et2Se2) has also been
reported as ALD precursor,[13,14] however its deposition is
generally H2-assisted and, therefore, less controlled. Recently,
tetrakis(N,N-dimethyldithiocarbamate)selenium (SDMDTC) was
successfully tested for deposition of Sb2Se3 by the reaction with
tris(dimethylamino)antimony (TDMASb).[15] This precursor seems
to be volatile enough within a range of 100 to 165 °C.
Bis(trialkylsilyl)selenides (R3Si-Se-SiR3) were introduced in
2009[16] and were demonstrated as suitable selenium precursors
for deposition of various metal selenides. Variation of the
R-substituent allowed tailoring volatility and reactivity towards
metal halides often used in ALD and provide repeatable
results.[17–19] However, linear bis(trialkylsilyl)selenides are
generally prone to quick hydrolysis and oxidation and their
handling is less comfortable. Hence, we report herein cyclic
silylselenides 1–3 that preparation is outlined in Scheme 1. The
facile synthesis utilizes in-situ generation of the lithium selenide
by the reaction of elemental selenium with Superhydride.
straightforward manner. Property tuning has been achieved by
varying the ring size and the number of embedded selenium atoms.
All silylselenides possess improved resistance towards moisture and
oxidation as well as high thermal robustness and sufficient volatility
with almost zero residues. Especially the six-membered diselenide
proved to be superior Se precursors for atomic layer deposition and
allowed facile preparation of MoSe2 layers. Their structure and
composition has been investigated by Raman and X-ray
photoelectron spectroscopy as well as scanning electron microscopy
revealing vertically aligned flaky shaped nanosheets.
Atomic layer deposition (ALD) belongs to a family of chemical
vapor deposition (CVD) techniques that attracts considerable
attention especially in thin-film deposition. It is mainly due to its
unique capability of manufacturing highly conformal nanolayers
on differently shaped substrates with desired film thickness. [1,2]
A
simplified ALD process involves a volatile (metallic) precursor,
which reacts with a selected substrate. The saturative process
allows a sole reaction of unoccupied functional groups of the
substrate leading to a deposition of one atomic layer. A
subsequent repetition after chamber purging allows to precisely
control the film thickness. A variety of materials can be deposited
in this way, e.g. metal chalcogenides that offer many interesting
properties across a wide variety of scientific fields. For instance,
well-studied CuInGaSe (CIGS) solar cells showed total
conversion efficiency exceeding 22 %[3] and durability/flexibility in
outdoor conditions. [4] Transition metal dichalcogenides (TMDs)
may accommodate sodium and magnesium ions, and, besides
energy production, found use as a promising electrode material in
the energy storage devices.[5] TMDs thin layers often show
Subsequent
treatment
of
Li2Se
with
1,2-
1
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bis(chlorodimethylsilyl)ethane
1,3-bis(chlorodimethylsilyl)propane
(n
=
=
2)
and
afforded
screened. In order to mimic an ALD process, their reactions in
dichloromethane afforded immediately dark precipitate, the initial
selenium precursor has completely been consumed and the
reaction contained only volatile dichlorosilanes (see the SI). Thus,
the cyclic silylselenide 1–3 showed very good trade-off between
stability and reactivity.
Subsequently, all three cyclic selenides were tested as Se
precursor in a custom thermal ALD system combined with MoCl5
as Mo precursor (experimental details provided in the SI). The
morphologies of the as deposited MoSe2 were characterized by
scanning electron microscopy (SEM) images. ALD processes
based on the use of cyclic selenides 1 and 3 exhibited abundant,
vertically aligned flaky shaped nanosheets, as displayed in Figure
2. These results agree well with the MoSe2 deposited by other
techniques.[22–27] Conversely, the six-membered cyclic selenide 2
showed poor reaction rate and negligible amount of material was
deposited. This is most likely due to its higher chemical stability
and thus resulting lower reactivity under the given deposition
conditions - flow reactor mode with time-limited contact between
the precursors and kinetically limited reaction
(n
3)
cyclosilylselenides 1 and 2 in high yields of 83 and 95 %,
respectively. The reaction with 1,2-dichlorotetramethyldisilane
provided crude 3,[20] which may be further purified by
crystallization or sublimation. Selenides 1[21] and 2 are viscose
liquids that can be easily purified by vacuum distillation.
Six-membered cycle 3 represents first solid ALD precursor with
more than one Se atom. Complete synthetic details and
characterizations are provide in the SI.
Scheme 1. Convenient synthesis of cyclic silylselenides.
Thermal behavior of 1–3 has been studied by TGA and DSC
methods, see Figure 1 and the SI. Whereas the TGA showed very
good volatility accompanied by complete evaporation and almost
zero residues, the complementary DSC measurements revealed
evaporation unaccompanied by
a decomposition. Thermal
properties of 1–3 are mostly dictated by the ring size and the
number of embedded Se/Si atoms. For instance, silylselenide 3
with two selenium atoms possesses the melting point of 145 °C,
which is close to the boiling points of 1–2 (170/200 °C), and
significantly shifted boiling point up to 275 °C. Despite high boiling
point of 3, TGA (Figure 1a) revealed its gradual
evaporation/sublimation starting at around 120 °C (~60 °C for 1–
2, measured at ambient pressure).
Figure 2. SEM top-view images at different magnification of MoSe2
nanostructures deposited on thin anatase TiO2 layer using MoCl5 and cyclic
selenide 1 (a,c) and 3 (b,d).
Surface characterization using Raman spectroscopy and X-ray
photoelectron spectroscopy (XPS) was carried out to confirm the
chemical composition of the deposited MoSe2. Raman
Figure 1. TGA (a) and DSC (b) records for 1 (blue), 2 (red) and 3 (green).
spectroscopy is
a paramount method for the structural
characterization of layered materials. Figure 3a-b shows the
corresponding Raman spectra of MoSe2. Characteristic lattice
vibrational modes of MoSe2 were observed, indicating
polycrystalline nature of the MoSe2. The distinctive peaks of
Moisture sensitivity and resistivity towards oxidation on air of silyl
selenides 1–3 has been examined by keeping their solution in
open vials. The NMR and GC/MS spectra (see the SI) have
indicated no decomposition up to 28 h; a very small amount of
elemental selenium has been deposited after 2 days. As
compared to previous linear bis(trialkylsilyl)selenides,[16] the
improved stability of 1–3 is attributed to cyclic arrangement,
which is less prone towards nucleophilic attack (e.g. hydrolysis)
or oxygen incorporation. Moreover, the stability is dependent on
the ring size and the content of the Se/Si atoms. Elemental
selenium and silanoles were identified as the main decomposition
products. A reactivity of cyclic selenides 1–3 towards metal
chlorides such as MoCl5 and ZnCl2 has preliminary been
MoSe2, A1g (out-of-plane) and E1 (in-plane) modes were
2g
observed for MoSe2 at ~239 and ~288/289 cm-1, respectively.
Compared to the peak positions for bulk MoSe2 (242 and 286 cm-
1), these lattice vibrational modes exhibited red and blue shift,
which indicates few-layered nature of the deposited MoSe2. In
addition, the peak at ~350 cm-1, assigned to the Raman-inactive
mode A2u points out few-layer 2H-TMDs, as such mode A2u gets
active as the crystal symmetry along the c-axis is lost.
Furthermore, the clear differences in the relative intensities
2
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between the A1g and E1 modes indicated the prevailing out-of-
during the aforementioned stability tests. The presence of Mo-Cl
species was further corroborated by the peak with binding energy
EB (2s)= 269.7 eV from the Cl 2s XPS spectrum (see Figure S11a
in the SI). These results indicated that i) the position of Mo(IV)–
2g
plane orientation of the MoSe2 nanosheets.[28]
Se matches well with previous works reported indicating 2H
[25,29,30]
MoSe2
and ii) undesired incomplete ligand exchange
reaction leading to residual Cl content. In order to prevent
undesired residual contents of ligand precursor, the optimization
of the ALD process is compulsory. On the other hand, the XPS
spectra of Mo 3d and Se 3d core levels corresponding to the as
deposited MoSe2 structures prepared by ALD using MoCl5 and
precursor 3 are showed in Figure 3e-f. The Figure 3e shows the
detailed analysis of the Mo 3d core levels. Therein, it is disclosed
a mixture of Mo(IV)-Se with binding energy EB (3d5/2) = 229.1 eV
and EB (3d3/2) = 232.2 eV, together with Mo(VI)-O with EB (3d5/2
= 232.7 eV and EB (3d3/2) = 233.8 eV, and Mo-O-Ti with EB (3d5/2
)
)
= 230.7 eV and EB (3d3/2) = 233.8 eV. The origin of the minor
presence of Mo(VI)-O and Mo-O-Ti species can be attributed to
MoSe2/Ti substrate interface effect. During the very early ALD
stage (initial ALD cycles), it could be expected that MoCl5 reacts
with adsorbed hydroxyl groups from the TiO2 substrate surface.
This reaction originates the chemical bonds Mo(VI)-O and Ti-O-
Mo, the latter furtherly corroborated by the Ti 2p XPS spectrum
from Figure S11b. On the other hand, the used Ti substrates
guarantee sufficient reactivity for the ALD reactions presented
here, which e.g. pure silicon could not easily offer. In the spectrum
of Se 3d core level (Figure 3f), the position of Mo(IV)–Se with EB
(3d5/2) = 54.6 eV and EB (3d3/2)=55.5 eV agree with previous works
reported indicating 2H MoSe2.[24,28 ,29]
A straightforward synthesis of cyclic silylselenides has been
developed. Compared to the limited portfolio of Se precursors, the
prepared selenides significantly extent the available selenium
compounds for ALD. These can be easily prepared, their
purification is very easy, offer facile structural/property tuning,
high thermal robustness and volatility, and generally good trade-
off between stability and reactivity. Two precursors showed
sufficiently fast reaction with metal precursors to allow their direct
application in ALD and deposition of MoSe2 nanostructures. The
solid cyclic silylselenide 3 bearing two selenium atoms seems to
be superior so far. A further structural elaboration on cyclic
selenides and optimization of the deposition experiments are
ongoing in our lab.
Figure 3. Raman spectra of the MoSe2 deposited using cyclic sylilselenide 1 (a)
3 (b). The main peaks of in-plane and out-of-plane modes are referred. High-
resolution XPS spectra of Mo 3d and Se 3d peaks corresponding to the MoSe2
deposited using cyclic silylselenide 1 (c-d) and 3 (e-f), respectively.
These results are in good agreement with the SEM images and
confirm the randomly aligned out-of-plane MoSe2 layers.
Acknowledgements
Surface compositional analyses of the MoSe2 deposited using
MoCl5 and Se precursor 1 (Figure 3c-d) and 3 (Figure 3e-f) was
performed using XPS. In Figure 3c, the deconvoluted spectrum of
Mo 3d core level reveals a mixture of Mo(IV)-Se with binding
energy EB (3d5/2) = 228.8 eV and EB (3d3/2) = 231.9 eV, Mo-Cl with
EB (3d5/2) = 229.7 eV and EB (3d3/2) = 232.8 eV. In addition, Se
3s peaks at 229.3 eV and 231.1 eV agree well with Mo-Se and
Se0 species, respectively, observed in Se 3d. In Figure 3d, the
deconvoluted spectrum of Se 3d core level confirms the presence
of Se0 with binding energy EB (3d5/2) = 55.5 eV and EB (3d3/2) =
56.4 eV, and Mo-Se with EB (3d5/2) = 54.3 eV and EB (3d3/2) = 55.2
eV. The presence of Mo-Cl and elemental Se0 are ascribed to
incomplete ligand exchange reaction of the MoCl5 and cyclic
sylilselenide 1 or its partial decomposition during the ALD process.
A minor deposition of elemental selenium has also been observed
This research was supported by the Czech Science Foundation
(18-03881S). R. Z., L. H., J. P., J. R.-P., and J. M. M. are indebted
to the Ministry of Education, Youth and Sports of the Czech
Republic (CZ.02.1.01/0.0/0.0/16_013/0001829).
Keywords: atomic layer deposition • MoSe2 • nanosheets •
selenium • transition metal dichalcogenides
[1]
[2]
[3]
R. L. Puurunen, J. Appl. Phys. 2005, 97, 121301.
M. Leskelä, M. Ritala, Angew. Chem. Int. Ed. 2003, 42, 5548–5554.
M. Powalla, S. Paetel, E. Ahlswede, R. Wuerz, C. D. Wessendorf, T.
Magorian Friedlmeier, Appl. Phys. Rev. 2018, 5, 041602.
M. Konagai, K. Kushiya, in Thin-Film Solar Cells, ed. Y. Hamakawa,
Springer, Berlin, Heidelberg, 2004, pp. 183–210.
[4]
3
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COMMUNICATION
[5]
[6]
[7]
T. Lu, S. Dong, C. Zhang, L. Zhang, G. Cui, Coord. Chem. Rev. 2017,
332, 75–99.
Y. Wu, M. Xu, X. Chen, S. Yang, H. Wu, J. Pan, X. Xiong, Nanoscale
2016, 8, 440–450.
S. Ng, M. Krbal, R. Zazpe, J. Přikryl, J. Charvot, F. Dvořák, L. Strizik, S.
Slang, H. Sopha, Y. Kosto, V. Matolin, F. K. Yam, F. Bureš, J. M. Macák,
Adv. Mater. Interfaces 2017, 4, 1701146.
[8]
[9]
A. Ambrosi, Z. Sofer, M. Pumera, Chem. Commun. 2015, 51, 8450–8453.
H. Tang, K. Dou, C. C. Kaun, Q. Kuang, S. Yang, J. Mater. Chem. A 2014,
2, 360–364.
[10] T. Yao, T. Takeda, R. Watanuki, Appl. Phys. Lett. 1986, 48, 1615–1616.
[11] W. Faschinger, P. Juza, S. Ferreira, H. Zajicek, A. Pesek, H. Sitter, K.
Lischka, Thin Solid Films 1993, 225, 270–274.
[12] R. Browning, N. Kuperman, R. Solanki, V. Kanzyuba, S. Rouvimov,
Semicond. Sci. Technol. 2016, 31, 095002.
[13] R. Kimura, M. Konagai and K. Takahashi, J. Cryst. Growth 1992, 116,
283–288.
[14] H. Fujiwara, H. Kiryu, I. Shimizu, J. Appl. Phys. 1995, 77, 3927–3933.
[15] N. Mahuli, D. Halder, A. Paul, S. K. Sarkar, Chem. Mater. 2019, 31,
7434–7442.
[16] V. Pore, T. Hatanpää, M. Ritala, M. Leskela, J. Am. Chem. Soc. 2009,
131, 3478–3480.
[17] M. Krbal, J. Přikryll, R. Zazpe, F. Dvořák, F. Bureš, J. M. Macák, Phys.
Status Solidi - Rapid Res. Lett. 2018, 12, 4–7.
[18] S. Yoo, C. Yoo, E. S. Park, W. Kim, Y. K. Lee, C. S. Hwang, J. Mater.
Chem. C 2018, 6, 5025–5032.
[19] J. H. Jeong, D. J. Choi, Mater. Sci. Semicond. Process. 2016, 54, 42–50.
[20] U. Herzog, G. Rheinwald, J. Organomet. Chem. 2001, 627, 23–36.
[21] U. Herzog, U. Böhme, Silicon Chem. 2003, 2, 77–92.
[22] J. Yao, B. Liu, S. Ozden, J. Wu, S. Yang, M. T. F. Rodrigues, K. Kalaga,
P. Dong, P. Xiao, Y. Zhang, R. Vajtai, P. M. Ajayan, Electrochim. Acta
2015, 176, 103–111.
[23] L. Ma, X. Zhou, L. Xu, X. Xu, L. Zhang, W. Chen, J. Power Sources 2015,
285, 274–280.
[24] V. K. Mariappan, K. Krishnamoorthy, P. Pazhamalai, S. Sahoo, S. J. Kim,
Electrochim. Acta 2018, 265, 514–522.
[25] Y. Huang, Y. E. Miao, J. Fu, S. Mo, C. Wei, T. Liu, J. Mater. Chem. A
2015, 3, 16263–16271.
[26] B. Kirubasankar, S. Vijayan, S. Angaiah, Sustain. Energy Fuels 2019, 3,
467–477.
[27] B. B. Wang, M. K. Zhu, I. Levchenko, K. Zheng, B. Gao, S. Xu, K.
Ostrikov, J. Cryst. Growth 2017, 475, 1–9.
[28] D. Kong, H. Wang, J. J. Cha, M. Pasta, K. J. Koski, J. Yao, Y. Cui, Nano
Lett. 2013, 13, 1341–1347.
[29] Z. Lei, S. Xu, P. Wu, Phys. Chem. Chem. Phys. 2016, 18, 70–74.
[30] X. Zheng, L. Yang, Y. Li, L. Yang, S. Luo, Electrochim. Acta 2019, 298,
663–669.
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Entry for the Table of Contents
Volatile, sufficiently reactive, and easy-to-synthesized solid Se precursor based on cyclic sylilselenide has been developed and
synthesized. It possesses good trade-off between chemical and thermal stability and reactivity towards MoCl5. It has further been
used for depositing MoSe2 nanosheets that structure has been confirmed by various techniques.
5
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6
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