Reversible photoswitching of stimuli-responsive Si(100) surfaces engineered with an assembled 1-cyano-1-phenyl-2-[4′-(10-undecenyloxy)phenyl]- ethylene monolayer

Article abstract of DOI:10.1039/b809037h

Si(100) surfaces were molecularly engineered by covalent linkage of a monolayer of two stilbene-based chromophores, either 1-cyano-1-phenyl-2- [4′-(10-undecenyloxy)phenyl]-ethylene or its chlorine derivative, 1-cyano-1-(4-Cl-phenyl)-2-[4′-(10-undecenyloxy)phenyl]-ethylene. The hybrid systems have been probed by monochromatized angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and atomic force microscopy (AFM) measurements. Results indicated robust covalent linkage of stilbene molecules to the functionalized substrate surfaces. AFM lithography and contact angle (CA) analysis confirmed that the adopted molecular architectures proved to be well-suited for reversible cis-trans photoswitching promoted by UV irradiation in the solid state. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b809037h

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Reversible photoswitching of stimuli-responsive Si(100) surfaces engineered
with an assembled 1-cyano-1-phenyl-2-[40-(10-undecenyloxy)phenyl]-ethylene
monolayer
*
Antonino Gulino, Fabio Lupo, Guglielmo G. Condorelli, Maria E. Fragala, Maria E. Amato
*
`
and Giuseppe Scarlata
Received 28th May 2008, Accepted 5th August 2008
First published as an Advance Article on the web 22nd September 2008
DOI: 10.1039/b809037h
Si(100) surfaces were molecularly engineered by covalent linkage of a monolayer of two stilbene-based
chromophores, either 1-cyano-1-phenyl-2-[40-(10-undecenyloxy)phenyl]-ethylene or its chlorine
derivative, 1-cyano-1-(4-Cl-phenyl)-2-[40-(10-undecenyloxy)phenyl]-ethylene. The hybrid systems have
been probed by monochromatized angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and
atomic force microscopy (AFM) measurements. Results indicated robust covalent linkage of stilbene
molecules to the functionalized substrate surfaces. AFM lithography and contact angle (CA) analysis
confirmed that the adopted molecular architectures proved to be well-suited for reversible cis–trans
photoswitching promoted by UV irradiation in the solid state.
simple chemical etching procedure used to obtain atomically flat
and chemically well-defined surfaces.8 Moreover, it was also
reported that it is possible to produce relatively flat Si(100)
surfaces, predominantly SiH2–terminated.9 Various strategies
can be adopted to anchor organic molecules to silicon. Among
them, hydrosilation of molecular builders, functionalized with
terminal multiple bonds, on hydrogen-terminated surfaces,
remains the most suitable owing to the largest potential
applications.10
Introduction
There is much current academic and industrial interest in the
development of molecular films utilized as memory devices,
sensors, switches and, in general, as stimuli-responsive materials
(SRMs).1,2 SRMs ideally undergo reversible changes in one or
more properties (structure, phase morphology, electrical,
magnetic and mechanical response, etc.) upon application/
removal of an external stimulus, such as a change in temperature,
ionic strength, pH, electric, magnetic or mechanical fields or by
chemical, optical, or biological analytes. Such materials have
potential smart applications in sensors, actuators, electro-optic
devices, etc. In this context, the control of molecular structure
and dynamics is of major importance from the perspective of
designing SRM-based devices. The combination of optically
responsive materials and monolayer assembly is a rapidly
emerging field because these materials are inexpensive,
miniature, robust and easy to fabricate.3 Engineering of inor-
ganic surfaces by covalent bonding of organic molecules also
represents an interesting approach to the synthesis of hybrid
inorganic–organic nanomaterials.4 In this field, for instance, the
large flexibility of porphyrins, fullerenes, and some inorganic
complexes has been recently exploited by some researchers in
order to assemble on SiO2 a variety of functional materials
capable of detecting various analytes by monolayer chemistry.5
In this field, well-organized molecular structures covalently
anchored to silicon6 or other conducting substrates,7 for facile
integration within electronic circuits, can be pursued, to build
molecularly based responsive materials in the perspective of
fabricating opto-electronic devices.
The photochemistry and photophysics of stilbene and its
derivatives have been extensively studied.11,12 Stilbene molecules
undergo photochemical cis–trans isomerization, dimerization,
and cyclization reactions in solution.11 Molecular switching
processes are crucial to the realization of devices that can operate
at both molecular and supramolecular levels.2m These stilbene
molecules allow reversible modulation of electronic properties by
external light triggering. Stilbene films have also been extensively
studied in order to exploit their solid-state isomerization
dynamics.12 These thin films are generally obtained by sublima-
tion,13 spin coating,14 or by the Langmuir–Blodgett15 (LB)
technique. With these techniques, the organization of the mole-
cules within the film is difficult to control. Moreover, the
resulting films are soluble in organic solvents. Recently, stilbene
monolayers have also been obtained by self-assembly.12b,e,f,h–j,l,m,o,p
This simple method relies on the covalent/non-covalent
anchoring of the molecule on various substrates and results in
great control of the molecular organization within the mono-
layer. Previous studies have revealed that the photochemistry of
stilbene in the solid state12b,g,j–l,n,o and in micelles11g might differ
from that in solution. In addition, the solid-state molecular
environment is relevant for creating the properties needed in the
photonic field. This interesting scenario prompted us to synthe-
size two novel cyano-stilbene systems with a molecular archi-
tecture that functions with covalent grafting on SiH2-terminated
Si(100) surfaces. This overall activity focuses on the development
of a platform that will pave the way for direct photo-switching in
Several recent studies on grafting of organic molecules on
silicon surfaces focused on the Si(111) substrate because of the
Dipartimento di Scienze Chimiche, Universita` di Catania and I.N.S.T.M.
UdR di Catania, Viale Andrea Doria 6, 95125 Catania, Italy. E-mail:
agulino@dipchi.unict.it; Fax: +39-095-580138
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the solid state, potentially useful for molecularly based infor-
mation storage materials on Si surfaces. The monolayer response
is based on AFM and contact angle monitoring.8a,c,11g Stable,
reproducible responses upon repeated and alternating light
radiation exposure has always been achieved with no degrada-
tion or material loss from the surface. Therefore, in this study,
we report on the switching properties of the solid-state device,
thus demonstrating the fabrication of a ‘‘photo-command’’
Si(100) surface.3a In addition, the synthesis, characterization, and
covalent assembly of the novel stilbenes, (Z)-1-cyano-1-phenyl-
2-[40-(10-undecenyloxy)phenyl]-ethylene, hereafter (Z)-UCS,
and its chlorine derivative, 1-cyano-1-(4-Cl-phenyl)-2-[40-(10-
undecenyloxy)phenyl]-ethylene, hereafter (Z)-UCS(Cl), are also
described.
(t, 2H), 7.36 (t, 1H), 6.99 (d, 2H), 5.79 (m, 1H), 4.94 (dd, 2H), 4.05
(t, 2H), 2.05 (dt, 2H), 1.81 (m, 2H), 1.51 (m, 2H), 1.38–1.32 (m,
10H). IR (KBr): n 2920, 2842, 2205, 1630, 1584, 1503, 1300, 1252,
1176, 905, 835 cmꢂ1. TOF SIMS m/z calcd for C26H31NO:
373.524; found ¼ 373.321. Anal. calcd: C, 83.55; H, 8.38; N, 3.77,
found C, 83.59; H, 8.32; N, 3.71. (Z)-UCS(Cl): melting point:
59–60 ꢃC (from EtOH). 1H NMR (EtOH-d6): d 7.96 (d, 2H), 7.75
(s, 1H), 7.70 (d, 2H), 7.45 (d, 2H), 7.01 (d, 2H), 5.78 (m, 1H), 4.94
(dd, 2H), 4.05 (t, 2H), 2.04 (dt, 2H), 1.81 (m, 2H), 1.51 (m, 2H),
1.28–1.42 (m, 10H). IR (KBr): n 2920, 2840, 2210, 1635, 1590,
1510, 1460, 1300, 1255, 1185, 1090, 910, 830 cmꢂ1. TOF SIMS m/z
calcd for C26H30ClNO: 407.975; found ¼ 407.968. Anal. calcd: C,
76.47; H, 7.41; Cl, 8.73, N, 3.43, found C, 76.53; H, 7.36; Cl, 8.91;
N, 3.49.
Experimental details
Monolayer formation and optical switching
Aldrich grade reagents and solvents, used throughout all present
syntheses, unless otherwise noted, were used as received. Solvents
for substrate cleaning were distilled. Elemental analyses were
performed using a Carlo-Erba 1106 model apparatus. Static TOF
SIMS (Time of Flight Secondary Ions Mass Spectrometry)
spectra were acquired in a static mode with a reflector-type
spectrometer (ION-TOF TOFSIMS IV), using a pulsed 69Ga+
primary ion beam (25 KeV, ꢀ0.1 pA) rastered over a 300 ꢁ
300 mm2 area. 1H NMR spectra were recorded using a 500 MHz
Varian Unity Inova spectrometer. Chemical shifts, which referred
to the residue signal of the deuterated solvent (EtOH-d6), were
independent of concentration effects within the experimental
concentration used (1–5 ꢁ 10ꢂ3 M). IR spectra were obtained with
a Perkin-Elmer 1340 spectrophotometer. The thermal behavior of
both (Z)-UCS and (Z)-UCS(Cl) was investigated by thermal
gravimetric analysis under atmospheric pre-purified nitrogen,
using a 2 ꢃC minꢂ1 heating rate in the 25–400 ꢃC range. A Mettler
Toledo TGA/SDTA 851 system was used. Samples of 6–8 mg were
accurately weighed and examined in the 25–400 ꢃC range.
For the fabrication of self-assembled monolayers, the Si(100)
substrate was first cleaned with ‘‘piranha’’ solution (c H2SO4 :
30% H2O2 70 : 30 v/v) at room temperature for 12 min, rinsed in
double distilled water for 2 min, etched in 2.5% hydrofluoric acid
for 90 s, washed with double distilled water for 20 s, accurately
dried with pre-purified N2, and immediately placed in a stilbene-
containing flask.10a,16 In particular, 25 mg of (Z)-UCS or
(Z)-UCS(Cl) were dissolved in 10 mL of anhydrous 1,3,5-trime-
thylbenꢃzene (mesitylene). The stilbene systems were then refluxed
at 160 C for 2 h, under slow N2 bubbling. After having been
cooled to room temperature, the substrates were removed from
the flask, rinsed, and repeatedly sonicated in dichloromethane,
pentane, and toluene to remove any residual unreacted stilbene.
This overall procedure has already proved to be effective in
obtaining covalent grafting of alkene-terminated molecules on
Si(100) surfaces.8,9,10a,16 A blank test performed using Si(100)
substrates not etched with HF, thus covered with an SiO2 over-
layer, gave no XPS evidence (vide infra) of grafted stilbene
molecules. The obtained monolayer-bearing substrates are
robust and remain intact, either after repetitive sonication, or
upon the scotch tape de-cohesion test,17 as evidenced by XPS
measurements (vide infra). Air equilibrated ethanol (Z)-UCS and
(Z)-UCS(Cl) solutions were irradiated (l ¼ 254 nm or 366 nm,
8 Watt) in quartz tubes, in a dark room, at room temperature to
promote cis–trans isomerization. Stilbene monolayers were
irradiated (l ¼ 254 nm or 366 nm, 8 Watt, 30 min each cycle).
Angle-resolved X-ray photoelectron spectra (AR-XPS) were
measured at different takeoff angles (5ꢃ, 10ꢃ, 20ꢃ, 45ꢃ, 80ꢃ), rela-
tive to the surface plane, with a PHI 5600 Multi Technique
System, which offers good control of the electron take-off angle
(base pressure of the main chamber 2 ꢁ 10ꢂ10 Torr).5,10a The
acceptance angle of the analyzer and the precision of the sample
holder concerning the takeoff angle are ꢄ3ꢃ and ꢄ1ꢃ, respectively.
The spectrometer is equipped with a dual anode X-ray source:
a spherical capacitor analyzer (SCA) with a mean diameter of
279.4 mm and an electrostatic lens system, Omni focus III.
Samples were mounted on Mo stubs. Spectra were excited with
monochromatized Al–Ka radiation. The XPS peak intensities
were obtained after Shirley background removal. No relevant
charging effect has been observed. Freshly prepared samples were
quickly transferred to the XPS main chamber. Experimental
uncertainties in binding energies lie within ꢄ0.28 eV.10a Some
Synthesis of 4-(10-undecenyloxy)benzaldehyde
4-Hydroxybenzaldehyde (0.5 g, 4.1 mmol), 1.14 g of 11-bromo-
1-undecene (4.92 mmol), and 0.102 g of 18-crown-6 (0.41 mmol)
were dissolved in 5 mL of acetone. After the addition of 0.85 g of
potassium carbonate (6.1 mmol), the reaction mixture was stirred
and refluxed for 24 h. Then, the mixture was cooled, filtered, and
concentrated in vacuum. The resulting oil was subjected to
column chromatography using CHCl3 to afford the product as
pale yellow oil (0.79 g, 2.9 mmol, 71% yield). 1H NMR (CDCl3):
d 9.88 (s, 1H), 7.83 (d, 2H), 6.99 (d, 2H), 5.82 (m, 1H), 4.96
(dd, 2H), 4.04 (t, 2H), 2.05 (dt, 2H), 1.82 (m, 2H), 1.47 (m, 2H),
1.28–1.42 (m, 10H).
Synthesis of (Z)-1-cyano-1-phenyl-2-[40-(10-undecenyloxy)phenyl]-
ethylene and (Z)-1-cyano-1-(4-Cl-phenyl)-2-[40-(10-
undecenyloxy)phenyl]-ethylene
(Z)-UCS and (Z)-UCS(Cl) were prepared by condensation of
stoichiometric quantities of phenylacetonitrile or 4-Cl-phenyl-
acetonitrile and 4-(10-undecenyloxy)benzaldehyde in ethanol
with EtONa. (Z)-UCS: melting point: 41–42 ꢃC (from EtOH). 1H
NMR (ethanol-d6): d 7.95 (d, 2H), 7.70 (d, 2H), 7.69 (s, 1H), 7.45
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spectra were deconvoluted by fitting experimental profiles with
a series of symmetrical Gaussian envelopes after subtracting the
background. The agreement factor, R ¼ [S(F0 ꢂ Fc)2/S ꢂ (F0)2]½,
after minimization of the function S(F0 ꢂ Fc)2 converged to R
values #0.04.
Results and discussion
The condensation reaction in alkaline medium between the
appropriate aldehyde and the arylacetonitrile derivative
produces cyanostilbenes in their Z configuration (>99%) (Fig. 1),
1
consistent with H-NMR spectra and both COSY and NOESY
Atomic force microscopy (AFM) measurements were
performed with a Solver P47 NTD-MDT instrument in semi-
contact mode (resonance frequency 150 Hz). The noise level
before and after each measurement was confined within
ꢄ0.01 nm. Atomic force lithography was performed by rastering
the AFM tip with a curvature radius of 10 nm under a constant
force of 0.25 mN.
2D map analyses. Air-equilibrated ethanol (Z)-UCS and
(Z)-UCS(Cl) solutions undergo reversible photo-isomerization
upon UV irradiation (l ¼ 254 or 366 nm) and changes are
apparent in NMR spectra (Fig. 2).11,12 In fact, upon illumination
at 366 nm, a new set of slightly upfield-shifted resonances, arising
from the aromatic, vinylic, and oxymethylene protons, become
apparent, consistent with the (E)-isomer formation.12n Moreover,
such an evolution parallels the fall-off of intensities due to the
(Z)-isomer. Importantly, no photocyclized derivatives or
photooxidized products have been detected by NMR spectros-
copy, after irradiation at room temperature. This behavior is in
good agreement with the presence of the electron-withdrawing
cyano substituent on the vinyl moiety.18 The 366 nm photosta-
tionary state shows a 1 : 3 (24% and 76%) (Z)-UCS : (E)-UCS,
isomer ratio. Moreover, the present experiments showed that the
(E)-UCS remains stable in the dark, at room temperature even
for prolonged periods (10 days) and, therefore, the reverse
(E)–(Z) isomerization can be photochemically switched. Irradi-
ation with the shorter 254 nm UV wavelength recovers the
(Z)-UCS isomer and the new 254 nm photostationary state
reveals a reversed 3 : 1 (75% and 25%) (Z)-UCS : (E)-UCS isomer
CA measurements were performed with a Kernco goniometer
10 min after the irradiation, at room temperature to exclude
thermal effects. Two mL water drops were applied on the sample
surface and measurements of the CA were made on both sides of
the two-dimensional projection of the droplet. Every measure-
ment was performed using 5 droplets on different surface
portions of the substrate in order to have statistically reliable
results. Differences in CA values of different droplets for every
set of measurements were confined to a few decimals.
Fig. 1 Molecular structure of (Z)-UCS and (Z)-UCS(Cl).
Fig. 3 TGA of (Z)-UCS (solid line) and (Z)-UCS(Cl) (dashed line)
showing no weight loss up to 208 ꢃC and 228 ꢃC, respectively.
Table 1 XPS atomic concentration analysis of the Si(100)-UCS(Cl)-
CAM. PTA ¼ photoelectron take-off angle
PTA/ꢃ
Si 2p
O 1s
21.8
21.0
19.2
17.7
17.0
C 1s
43.4
45.0
57.0
62.8
66.5
N 1s
1.4
1.5
1.9
2.2
2.6
Cl 2p
80
27.8 (Si) + 4.1
(SiO2) ¼ 31.9
25.6 (Si) + 5.3
(SiO2) ¼ 30.9
14.0 (Si) + 5.8
(SiO2) ¼ 19.8
8.8 (Si) + 6.2
(SiO2) ¼ 15.0
3.1 (Si) + 8.3
(SiO2) ¼ 11.4
1.5
45
1.6
20
2.1
Fig. 2 1H-NMR spectra in the 3.4–4.3 ppm range. Lines (a), (b), (c) and
(d) refer to UCS. Lines (e), (f), (g) and (h) refer to UCS(Cl). Specifically, (a)
and (e) as synthesized (Z)-isomers; (b) and (f) after 10 min of irradiation at
366nm; (c) and (g) after 30 min of irradiation at 366 nm; (d) and (h) after 30
min of irradiation at 254, nm subsequent to irradiation at 366 nm.
10
2.3
5
2.5
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Fig. 4 Monochromatized Al Ka excited XPS of Si(100)-UCS(Cl)-CAM in (a) the Si 2p energy region; (b) the C 1s energy region (the dashed line,
superimposed to the experimental profile, refers to the sum of the Gaussian components); (c) N 1s energy region.
ratio (Fig. 2(a)–(d)). NMR measurements provide evidence that
irradiation times in the 1–30 min range force a progressive isom-
erization, whereas prolonged times (up to 1 h) do not significantly
affect NMR spectra, as a consequence of the established trans–cis
photostationary equilibrium. Photoisomerization experiments
involving UCS(Cl) show a more efficient conversion towards a cis
phostationary state (366 nm) with an 18% (Z) and 82% (E) ratio
between the two isomers (Fig. 2(e)–(g)).
that afforded covalent grafting of the stilbene units.10a,16 Both
Si(100)-supported (Z)-1-cyano-1-phenyl-2-[40-(10-undecenyloxy)
phenyl]-ethylene covalently assembled monolayer, hereafter
Si(100)-UCS-CAM, and its chlorine derivative, (Z)-1-cyano-1-
(4-Cl-phenyl)-2-[40-(10-undecenyloxy)phenyl]-ethylene, hereafter
Si(100)-UCS(Cl)-CAM, have been unambiguously characterized
by X-ray photoelectron spectroscopy measurements.
This technique appears ideally suited since it provides high
vertical resolution, using angle-resolved measurements, and gives
However, the reverse isomerization, to restore the trans state, is less
efficient and results in 38% of (Z)-UCS(Cl) and 62% of (E)-UCS(Cl)
(Fig. 2(h)). Upon switching between the 366 nm and 254 nm radia-
tions, the ratio is 38–40% (Z) and, as a consequence, 62–60% (E).
The high thermal stability of both (Z)-UCS and (Z)-UCS(Cl)
has been determined by thermal gravimetric analysis (TGA).
(Z)-UCS remains stable up to 208 ꢃC (Fig. 3). Beyond this range,
it sublimates with an 8.1% residue left at 350 ꢃC. Similar results
have been obtained for (Z)-UCS(Cl), whose sublimation begins
at 228 ꢃC (Fig. 3). Both systems were found to be thermally stable
beyond the tempꢃerature required for the synthesis of related
monꢃolayers (160 C). In addition, both systems were heated at
160 C for 2 h and then their NMR spectra checked again. No
decomposition at all was observed. Self-assembled monolayers
were synthesized using an already optimized one-step procedure
Fig. 6 AFM image (semi-contact mode) and cross section of a repre-
Fig. 5 Schematic representation of Si(100)-UCS-CAM.
5014 | J. Mater. Chem., 2008, 18, 5011–5018
sentative Si(100)-UCS-CAM.
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information on the bonding states of the grafted molecules.5,19 In
this context, the presence of the additional Cl heteroatom on
the chlorine derivative allows better characterization. Therefore,
AR-XPS data of prototypical Si(100)-UCS(Cl)-CAM are here
discussed.20,21 Accumulating evidence suggests a decrease in both
Si and O peaks, with a sizeable opposite trend for C, N, and Cl
signals, at low photoemission angles (Table 1). In particular, the
whole Si 2p signal (Fig. 4a) strongly decreases from 31.9 at 80ꢃ to
11.4% at a 5ꢃ photoelectron takeoff angle (PTA). The Si 2p feature
is resolved into two components at 99.0 (2p3/2) and 99.6 (2p1/2) eV
with a spin–orbit separation of 0.6 eV, in accordance with that
expected for a Si(100) substrate.10a The broad band at 103.1 eV is
consistent with the presence of some SiO2.10a Therefore, the
decreasing intensity trendof the whole Si 2p signal on a decrease of
PTA is largely due to the fall-off of the Si(100) component since
the SiO2 peak, in contrast, shows a small increase. This overall
observation points to some expected substrate surface oxidation
already detected in similar systems.10a Analogously, the O 1s
signal at 533.0 B.E. eV suffers an intensity decrease from 21.8
(at 80ꢃ) to 17.0% (at 5ꢃ).8–10 Photoelectron spectra in the C 1s
region (Fig. 4b) show a rich structure clearly because of several
bonding states. There are three main components centered: i)
at 285.0 eV due to both aliphatic and aromatic backbones,22 ii)
at 286.4 eV due to the carbon centers of the stilbene molecule
bonded to the oxygen atom,23 and, iii) at 287.4 eV due to the
carbon of the –CN group.23 Notably, the whole carbon atomic
concentration significantly increases (43.4% vs. 66.5%) when
passing from 80ꢃ to 5ꢃ PTA, with a more remarkable contribution
of the higher (287.4 eV) energy component. Finally, the N 1s
spectrum (Fig. 4c) exhibits a broad peak at 400.0 eV. This feature
accounts for the nitrogen of the cyano group.10a The nitrogen
atomic concentration also exhibits an appreciable increase upon
decreasing the electron take-off angle from 1.4% (at 80ꢃ) to 2.6%
(at 5ꢃ), as expected for the upper layer nature of the signal.
Notably, the nitrogen : chlorine atomic concentration ratio
always well matches the theoretical 1 : 1 value.
The film thickness, estimated using a model for an ideal planar
structure with a homogeneous carbonaceous film of thickness d,
Fig. 7 AFM images (semi-contact mode) and cross sections of a representative Si(100)-UCS-CAM upon alternating AFM lithography vs. 30 min UV
irradiation: (a) upon 366 nm, and (b) upon 254 nm irradiation; and (c) cycling irradiation and AFM measurements.
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is 18 A12g thus suggesting an inclined (ca. 45ꢃ) grafting geometry
The photoisomerization reaction of the present monolayer
system on the Si(100) surface was also investigated by alternating
irradiation at 366 and 254 nm with aqueous CA analysis.11g,12a,c,p
Fig. 8 shows the monolayer performance/stability of the revers-
ible photo-isomerized states demonstrated for 8 irradiation
cycles.
˚
(Fig. 5) already observed in similar Si(100)-bonded monolayers
(vide infra).16 The estimated surface coverage with (Z)-UCS(Cl)
molecules results in 8 ꢁ 1013 molecules cmꢂ2 with a footprint of
150 A per stilbene unit.5b,21 In summary, the XPS technique has
˚
provided useful details particularly rich in unique chemical
information not accessible using other, alternative spectroscopic
techniques.
The system remains stable under the adopted conditions.
No hysteresis, relevant drift, or change in the output signal,
independent of the input, has been observed for prolonged
storage of the system in a dark room, at room temperature, after
irradiation. Both photo-isomerized states remain stable in air for
prolonged periods of time. The stilbene molecules immobilized
on surfaces could suffer some steric hindrance that may partially
hamper the reversible cis–trans photo-switching.12b The XPS-
derived molecular footprint already excludes these interactions
for the present Si(100)-UCS-CAM. Nevertheless, additional
cleaned Si(100) substrates were immersed in a mesitylene solu-
tion containing a 1 : 9 (mol/mol) mixture of (Z)-UCS and decene,
for the fabrication of diluted monolayers. It has already been
demonstrated that the use of a 1-decene spectator spacer repre-
sents a suitable expedient for fabricating covalently assembled
monolayers having a composition/dilution strongly reminiscent
of that in solution.16 XPS results (N 1s/C 1s band intensity) of the
present mixed monolayer are indicative of 0.7 : 9 UCS : decene
molecules on this ‘‘diluted’’ Si(100)-UCS-CAM. C.A. measure-
ments on this system yielded variations similar to those already
observed for the ‘‘undiluted’’ monolayer.
Fig. 6a shows a representative AFM image of the Si(100)-
UCS-CAM. The surface morphology, obtained in semi-contact
mode, appears homogeneous with a mean roughness of 0.088 nm.
The thickness of the monolayer has been evaluated using AFM
lithography (contact mode).24 Thus, grafted molecules were
removed along straight lines by rastering two different areas (both
0.5 ꢁ 1 mm in size) of the surface with the AFM tip under a suitable
constant force. The two obtained scratch depths (Fig. 6b) are
almost coincident and equal 1.7 ꢄ 0.1 nm. This value approaches
that obtained by XPS for the Si(100)-UCS(Cl)-CAM.
Dynamic surface changes owing to the cis–trans isomerization
reaction can be monitored by AFM.1c The monolayer thickness
variations in the Si(100)-UCS-CAM have been determined by
alternating atomic force lithography vs. UV irradiation. Fig. 7a
shows the observed thickness changes as a result of 366 nm light
stimulus, followed by 254 nm (Fig. 7b) regeneration. The results
indicate that the cis isomerization produces a 25 ꢄ 5% decrease in
thickness, followed by almost regeneration of the starting
thickness upon irradiation at 254 nm. Taking into account the
inclined grafting geometry already obtained by XPS measure-
ments (vide supra), the observed thickness variation is in good
agreement with results of MM + calculations on the two isomers
Conclusion
Both thickness variations and contact angle measurements
unambiguously demonstrated the cis–trans solid state isomeri-
zation reaction of the new stilbene-based Si(100)-UCS-CAM,
promoted by UV irradiation. Angle-resolved XPS measurements
provided evidence that stilbene molecules were grafted on Si(100)
substrates. In addition, two new stilbene systems, namely,
1-cyano-1-phenyl-2-[40-(10-undecenyloxy)phenyl]-ethylene and
its chlorine derivative, 1-cyano-1-(4-Cl-phenyl)-2-[40-(10-unde-
cenyloxy)phenyl]-ethylene, were synthesized and characterized.
Finally, all these results indicate that this system is a good
candidate for fabricating SRMs on silicon and, in general, is
potentially useful for molecular-based devices that need direct
photo-switching in the solid state.
˚
˚
whose sizes, (Z) (24.8 A) and (E) (19.6 A), differ by 21%. Cycling
irradiation and AFM measurements (Fig. 7c) confirm reversible
switching surface modifications. This observation is in agreement
with literature data showing that the interfacial properties are
defined by the molecular-level structure of the surface and can be
monitored by AFM and CA analysis.1c
Acknowledgements
The authors thank NATO (SfP project 981964) and the Minis-
`
tero Universita e Ricerca (MUR, Roma) for financial support
(PRIN 2005 and FIRB 2003 – RBNE033KMA). Dr Alessio
Mamo is acknowledged for CA measurements. Dr M. E. van der
Boom (Weizmann, Israel) is fully acknowledged for the helpful
discussion of the results.
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