Thin films of organic molecular crystals (OMC) possessing type B lattice: spatial structure of dibenzotetraazaannulene film is related to its thickness
Diffractometric and optical measurements showed that at considerable (>200 oK) difference between sublimation temperature and substrate one dibenzotetraazaannulene forms well oriented films structure of which depends on their thickness. This phenomena was explained by assuming that the temperatur...
Gespeichert in:
| Datum: | 1999 |
|---|---|
| Hauptverfasser: | , |
| Format: | Artikel |
| Sprache: | English |
| Veröffentlicht: |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
1999
|
| Schriftenreihe: | Semiconductor Physics Quantum Electronics & Optoelectronics |
| Online Zugang: | http://dspace.nbuv.gov.ua/handle/123456789/119863 |
| Tags: |
Tag hinzufügen
Keine Tags, Fügen Sie den ersten Tag hinzu!
|
| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Zitieren: | Thin films of organic molecular crystals (OMC) possessing type B lattice: spatial structure of dibenzotetraazaannulene film is related to its thickness / B.A. Snopok, Ya.D. Lampeka // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 2. — С. 69-72. — Бібліогр.: 12 назв. — англ. |
Institution
Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
irk-123456789-119863 |
|---|---|
| record_format |
dspace |
| spelling |
irk-123456789-1198632017-06-11T03:03:17Z Thin films of organic molecular crystals (OMC) possessing type B lattice: spatial structure of dibenzotetraazaannulene film is related to its thickness Snopok, B.A. Lampeka, Ya.D. Diffractometric and optical measurements showed that at considerable (>200 oK) difference between sublimation temperature and substrate one dibenzotetraazaannulene forms well oriented films structure of which depends on their thickness. This phenomena was explained by assuming that the temperature gradient inside the film can be changed during the evaporation. Therewith, both diffusion processes in clusters and the restructurization of small crystallites were suggested to be important components of OMC film formation mechanism. 1999 Article Thin films of organic molecular crystals (OMC) possessing type B lattice: spatial structure of dibenzotetraazaannulene film is related to its thickness / B.A. Snopok, Ya.D. Lampeka // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 2. — С. 69-72. — Бібліогр.: 12 назв. — англ. 1560-8034 PACS 61.10.-i, 61.66.-f, 68.55.-a. http://dspace.nbuv.gov.ua/handle/123456789/119863 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| description |
Diffractometric and optical measurements showed that at considerable (>200 oK) difference between sublimation temperature and substrate one dibenzotetraazaannulene forms well oriented films structure of which depends on their thickness. This phenomena was explained by assuming that the temperature gradient inside the film can be changed during the evaporation. Therewith, both diffusion processes in clusters and the restructurization of small crystallites were suggested to be important components of OMC film formation mechanism. |
| format |
Article |
| author |
Snopok, B.A. Lampeka, Ya.D. |
| spellingShingle |
Snopok, B.A. Lampeka, Ya.D. Thin films of organic molecular crystals (OMC) possessing type B lattice: spatial structure of dibenzotetraazaannulene film is related to its thickness Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Snopok, B.A. Lampeka, Ya.D. |
| author_sort |
Snopok, B.A. |
| title |
Thin films of organic molecular crystals (OMC) possessing type B lattice: spatial structure of dibenzotetraazaannulene film is related to its thickness |
| title_short |
Thin films of organic molecular crystals (OMC) possessing type B lattice: spatial structure of dibenzotetraazaannulene film is related to its thickness |
| title_full |
Thin films of organic molecular crystals (OMC) possessing type B lattice: spatial structure of dibenzotetraazaannulene film is related to its thickness |
| title_fullStr |
Thin films of organic molecular crystals (OMC) possessing type B lattice: spatial structure of dibenzotetraazaannulene film is related to its thickness |
| title_full_unstemmed |
Thin films of organic molecular crystals (OMC) possessing type B lattice: spatial structure of dibenzotetraazaannulene film is related to its thickness |
| title_sort |
thin films of organic molecular crystals (omc) possessing type b lattice: spatial structure of dibenzotetraazaannulene film is related to its thickness |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
1999 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/119863 |
| citation_txt |
Thin films of organic molecular crystals (OMC) possessing type B lattice: spatial structure of dibenzotetraazaannulene film is related to its thickness / B.A. Snopok, Ya.D. Lampeka // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 2. — С. 69-72. — Бібліогр.: 12 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT snopokba thinfilmsoforganicmolecularcrystalsomcpossessingtypeblatticespatialstructureofdibenzotetraazaannulenefilmisrelatedtoitsthickness AT lampekayad thinfilmsoforganicmolecularcrystalsomcpossessingtypeblatticespatialstructureofdibenzotetraazaannulenefilmisrelatedtoitsthickness |
| first_indexed |
2025-07-08T16:48:34Z |
| last_indexed |
2025-07-08T16:48:34Z |
| _version_ |
1837098227276447744 |
| fulltext |
69© 1999, Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Semiconductor Physics, Quantum Electronics & Optoelectronics. 1999. V. 2, N 2. P. 69-72.
PACS 61.10.-i, 61.66.-f, 68.55.-a.
Thin films of organic molecular crystals (OMC)
possessing type B lattice: spatial structure of
dibenzotetraazaannulene film is related
to its thickness
B. A. Snopok*, Ya. D. Lampeka
* Institute of Semiconductor Physics of NASU, 45, prospect Nauki , 252028 Kiev, Ukraine
E-mail: snopok@isp.kiev.ua, Telephone: +380 (44) 265 56 26; 1 Phone (Voice&Fax): +380 (44) 265 18 27
L.V.Pisarzhevskij Institute of Physical Chemistry, NAS Ukraine, 31 prospect Nauki, Kyiv, 252039, Ukraine
Abstract. Diffractometric and optical measurements showed that at considerable (>200 oK) difference
between sublimation temperature and substrate one dibenzotetraazaannulene forms well oriented films
structure of which depends on their thickness. This phenomena was explained by assuming that the
temperature gradient inside the film can be changed during the evaporation. Therewith, both diffusion
processes in clusters and the restructurization of small crystallites were suggested to be important com-
ponents of OMC film formation mechanism.
Keywords: molecular crystals, thin films, growth mechanism, organic semiconductors, thermovacuum
deposition, layered solids, type B lattice.
Paper received 28.04.99; revised manuscript received 09.07.99; accepted for publication 12.07.99.
1. Introduction
Considerable study in the past decades is being given to the
mechanisms of formation of semiconductor surfaces because
the search for the ways to control their structure is of great
importance. Thin films of organic molecular crystals (OMC)
fabricated by means of classical for semiconductor micro-
electronics thermovacuum deposition seem to be especially
interesting. Such objects are perspective materials for devel-
opment of a new generation of electronic devices [1]. Present
work is devoted to studying a thin film structure of a new
organic semiconductor, dibenzotetraazaannulene, in an at-
tempt to specify the main features of their formation.
2. Experimental
The dibenzotetraazaannulene (Fig.1), 5,14-dihydrodiben-
zo[b,i][1,4,8,11]tetraazacyclotetradecine (H2TAA), was
synthesized according to [2] and recrystallized twice from
dimethylformamide. Films of above composition were fab-
ricated via thermovacuum deposition (ca. 5⋅10-4 Pa) on
melted quartz substrates. The thickness of films (d) was de-
termined using a microinterferometer (MII-4) and an ellipso-
meter (LEF-3M). Diffractograms were taken with diffrac-
tometer DRON-3M (Cu Kα radiation, λ = 1.54Å). The vis-
ible reflectance spectra were obtained using spectropho-
tometer SPECORD-M40 equipped with photometric sphere.
The presence of lustre trap allowed to determine both the
total reflection coefficient R and its diffuse component RD.
The mirror component was calculated as RM = R - RD. The
fractal dimension of films was determined according to [3].
3. The approximation of stratified structure
of thin films
In the present work the concept of stratified crystal growth
[4] is used, in which the normal growth of the crystal is
described by consecutive building-on of the layers. In such
an approximation some information about growth proces-
ses can be obtained from comparative analysis of characteris-
tics of films with different thickness. Therewith, the appli-
cability of this approximation is limited by condition of for-
mation of an ordered structure at least along the stratifica-
tion direction.
For the study of the film structure along the normal to
the substrate, the films were investigated by means of X-ray
B. A. Snopok, Ya. D. Lampeka: Thin films of organic molecular crystals...
70 SQO, 2(2), 1999
diffractometry (Fig.2). One can see that only two peaks are
observed on diffractograms of films, in contrast to more
than twenty ones characteristic of polycrystalline powder.
This evidences the presence of only two sets of reflecting
planes at 4.5 (19o 33') and 9.1 (9o 41') Å in the former case.
In fact, such feature indicates the formation of oriented thin
films of H2TAA. This is, apparently, the result of enhanced
intermolecular interaction in its crystal, which is typical to
the type B crystal lattice [5] with two pairs of parallel mole-
cules in an elementary unit [6] (Fig.1).
4. Origination of crystallization centers.
Considering that the substance is deposited on the plane
amorphous substrate from gaseous phase, one can believe
that the adsorption of H2TAA molecules (process 1, Fig.3)
proceeds stochastically. Before the beginning of crystalliza-
tion, the adlayer molecules diffuse freely along the surface
(on the average, during the time τ with diffusion coefficient
D, process 3). The formation of the solid film begins with
spontaneous origination of the complexes of adlayer mole-
cules which lost mobility owing to their collision (process
4). At least for two reasons, the spreading of such com-
plexes along the surface can proceed only owing to the addi-
tion of molecules from the adlayer. Firstly, the area of the
nuclei is much smaller than the area of substrate. Secondly,
the lattice breakage on the boundary of small clusters leads
to the enhancement of polarization energy forming in such
manner, the repulsive potential which prevent from the
adsorption (process 5). The latter effect must also take place
on boundaries of large clusters (process 5'). It follows from
this, in particular, that all the nuclei have a similar convex
shape, comparable dimensions and rates of growth. The aver-
age nucleus dimension <r> is determined by the diffusion
path <l> of the adlayer molecules: <r> ≈ <l> = (4⋅D⋅τ)1/2.
Since both τ and D depend on temparature, then <r> ∼ f(T).
The last fact is well established for OMC when only the
influence of substrate temperature Ts is considered [7].
5. The mechanism of crystal growth
As it follows from Fig.2, the variations of diffraction peak
intensities depending on film thickness indicate the appear-
ance of isolated «pseudomonocrystalline» clusters on early
stages of film formation. They are spreaded with the re-
tension of their ordering right up to dcrit ≈ 260÷270 nm. It is
felt that the substance at this film thickness completely cover
the substrate with ordered layer. Further thickening of films
leads to formation of stochastically oriented crystallites. It
is evident from both the weakening of diffraction peaks and
their broadening, as well as from the appearance of new
peaks at 2.5, 3.4, 4.1 and 6.3 Å, typical of polycrystalline
sample. The same conclusion can be drawn from optical
properties of the films (Fig.4). The increase of diffuse re-
flectance with thickening films indicates the rise of spatial
heterogeneity <l> of the samples which is proportional to
the average microcrystallite dimensions <r> .
All above mentioned allows to suggest that after reaching
Fig.1. Diagram of the unit cell contents of H2TAA (a), view illustrating
predimeric pairs of H2TAA in solid state (b) and projection viewed per-
pendicular to the ab-face (c) � showing the molecular stacking (recon-
struction based on the data [6] using MolDraw).
a
b
c
3.3A
3.9A
3.3 Å
3.9 Å
B. A. Snopok, Ya. D. Lampeka: Thin films of organic molecular crystals...
71SQO, 2(2), 1999
some definite dimension of surface clusters further growth
of the film along the normal to substrate proceeds owing to
the adsorption on the nuclei of molecules from gaseous phase
(process 1'). In its turn, the adhesion of «hot» molecules can
cause the change of temperature distribution inside the film
and thereby the increasing τ in the region of crystallization.
In order to prove this suggestion, the dependence of
T vs d was obtained by using a simple model [8]. Let us
consider the deposition process in quasistatical approxi-
mation when the heat transfer rate by gaseous molecules is
much smaller than the heat dissipation rate. The heat trans-
fer by molecules from gaseous phase may be modelled by
heat source with the power proportional to (Th - T) disposed
at x = d (Th is sublimation temperature). The solution of the
equation of heat transfer in such a model at boundary con-
ditions T=Ts (x = 0) and T = Th (x = d) is:
)}exp())2({exp(
)2exp(1
)(
ddx
d
TT hs ⋅−−⋅−⋅⋅
⋅⋅−−
−
− ββ
βhT=T
where β is some constant. It follows from the dependences
of T vs d that the temperature of near the surface layer rises
regularly with increasing d. Thus, the increase of average
crystallite dimensions with thickening the film may be
consistently explained by changing temperature gradient
inside the film. Since the formation of next layer is possible
only on completely crystallized surface, the restructuriza-
tion of defects (potential well of polarization energy) must
be of importance (process 6).
6. The interaction of crystallites
Taking into consideration the great difference between the
H2TAA sublimation temperature (≈520 K) and the temper-
ature of substrate (297 oK), the «drop» model of crystalli-
zation can not be applied to the system under consideration.
We suggest that the contact of two growing crystallites caus-
es the cessation of their growth in contact region but the
growth is continued in the rest directions. As a result, the
polycrystalline object consisting of the multitude of grains
having an irregular structure is formed.
Really, as for the diffusion, at d<dcrit the splicing of the
crystallites promotes much rapid increase of <l> (Fig.4)
that may be expected if only changing the temperature gra-
dient inside the film is accounted for (<l> ≥ <r> ). At
d ≈ dcrit the condition <l> ≈<r> is no longer valid since the
average diffusion displacement becomes much smaller than
the geomertrical dimensions of the sample owing to effec-
tive splicing of crystallites. In such a case, diffusion move-
ment leads to the appearance of spatial inhomogeneities with
typical dimensions <l> and causes formation of disordered
structure (<l> ≤ <r> ) at d >dcrit.
In the above discussion the only intercrystalline inter-
action taken into consideration was the cessation of the
growth of two crystallites in their contact region. In fact,
9
11.5
14
16.5
19
21.5
24
26.5
29
0
50
100
150
200
I
d
θ
Fig.2. X-ray diffraction patterns of H2TAA thin films (rising d = 85, 225, 260, 360, 1500 nm, powder).
Fig.3. The qualitative model of processes during the growth of films (1,
1'-adsorption, 2, 2'-desorption, 3-diffusion along substratum, 4-comple-
xation, 5, 5'-restructurization, 6-reorganization, 7-diffusion along the clus-
ter; different labels correspond to different energetic and absorption states
of molecules).
¬
¯
¯
¬
ÃÇ
É
ÈÄ ð ð
¬
Ãǯ¯
®®
ï
1 2 3 4
5
67
llllllllllllll
llllllllllll
lllllllllllll
llllllllllllll
llllllllll llll
lll
¬
¬
¬
¬
įÈ
2′ 1′ 5′
1
1
43
5
5
67
2
2
B. A. Snopok, Ya. D. Lampeka: Thin films of organic molecular crystals...
72 SQO, 2(2), 1999
however, the picture must be more complicated. The for-
mation of oriented H2TAA films and the increase of diffuse
reflectance with their thickening indicate the stochastic for-
mation of initial nuclei and the development of crystalliza-
tion front without origination of new nuclei. Physically, this
means that small crystallites are reorganized because of their
lesser thermodynamic stability [9]. Such a conclusion agrees
well with the measurements of film fractal dimensions DH.
Experimental values of DH = 2±0.05 are in good agreement
with values resulting from the model of percolation clus-
ters formation in this film [10]. It follows from computer
simulations that DH=2 can be obtained only when both the
reevaporation of small crystallites and the diffusion pro-
cesses inside clusters are taken into account in the mecha-
nism of film formation [11, 12].
References
1. F. Vogtle. Supramolecular Chemistry, London: John Wiley & Sons.
(1993).
2. H. Hiller, P. Dimroth, H. Pfitzner. 5,14-dihydro-dibenzo[b,i]
[5,9,14,18]tetraaza[14]annulen, ein makrocyclischen chelat-bildner.
Liebigs. Ann. Chem.,717, pp. 137-147 (1968).
3. C.Allain, M.Cloitre. Fractals in Physics. Proc. 6th Int. Symp., Amster-
dam-New York: North-Holland. pp. 91 (1986).
4. V.Z. Belenkiy. Geometrico-probability models of crystallization:
phenomenological description, Moscow: Nauka (1980) (in Russian).
5. B. Stevens. Effect of molecular orientation on fluorescence emission
and energy transfer in crystalline aromatic hydrocarbons. Spectro-
chim. Acta., 19, pp. 439-448 (1962).
6. E. Sister, V. Gottfried, M. Kapon, M. Kaftory, Z. Dory, H.B. Gray.
Structural characterization of the product of oxidation of a macrocy-
clic cobalt(II) complex in pyridine solution. Inorg. Chem., 27 (4),
pp. 600-604 (1988).
7. M. Yudasaka, M.Kawai, S.Kurita, K. Nakanishi, Y. Kuwae. Thin Solid
Films., 151, pp. L115 (1987).
8. B. A. Snopok, Ya. D. Lampeka. About the structure and formation
mechanism of thin films of dibenzotetraazaannulene on amorphous
substrate. Theoret. and Experim. Chem., 31 (6), pp.365-369 (1995).
9. E. Ruchenstein, B. Nowakowski. Langmuir., 8, pp.1470 (1992).
10. B.A. Snopok, Ya.D. Lampeka. Non-linear electrophysical properties
of thin films of dibenzotetraazaannulenes: Application of percolation
model. Mol. Cryst. Liq. Cryst., 242, pp.171-177 (1994).
11. P. Meakin, R. Jullien. The growth of self-affine fractal surfaces. Phys.
Rev., 41, pp. 983-993 (1990).
12. B.M. Smirnov. Physics of Fractal Aggregates. Moscow: Nauka (1991)
(in Russian).
Fig.4. The relationship between mirror (1,3) and diffusion (2,4) reflection
in strong (1,2) and weak (3,4) light absorption regions.
5
4
3
2
1
0
1 3
2 4
0 300 600 900 1200 1500
d, nm
20
15
10
5
0
R, % R, %
|