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Solid State Sciences xx (2007) 1e5
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Veronica Mugnaini a,1, Mauro Fabrizioli b,2, Imma Ratera a,1, Matteo Mannini b,2,
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Andrea Caneschi b,2, Dante Gatteschi b,2, Yishay Manassen c,3, Jaume Veciana a,*
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a
Institut de Ciencia de Materials de Barcelona e CSIC, Campus de la UAB, 08193 Bellaterra, Spain
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Laboratory for Molecular Magnetism and INSTM Research Unit, University of Florence, 50019 Sesto Fiorentino, Italy
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c
Department of Physics, Ben Gurion University of the Negev, P.O. Box 653, Beer Sheva 84105, Israel
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Received 6 June 2007; received in revised form 20 September 2007; accepted 28 September 2007
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Abstract
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The Electron Spin Noise Scanning Tunnelling Microscopy (ESN-STM) technique is one of the most promising techniques detecting one
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single spin, combining the spatial resolution of the STM with the ability of spectral resolution and spin manipulation of Electron Spin Resonance. After its first observation, the effectiveness of this technique has been tested by different groups to study the properties of small aggre26
gates or single organic radical molecules.
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We report on the ongoing ESN-STM study on nano-aggregates of tris(2,4,6-thrichlorophenyl)methyl radical (TTM) derivatives, whose pos28
sibility to be used as wires, switches and memory devices has already been investigated in bulk. After deposition on Au(111), TTM radicals
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preserved their magnetism. To treat the collected noise data we followed a statistical approach; some peculiar characteristics of this analysis
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will be addressed.
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Ó 2007 Elsevier Masson SAS. All rights reserved.
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Keywords: Single spin detection; STM; Organic radicals; CW ESR; Molecular magnetism
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on different instrumental set ups using well known ESR stan1. Introduction
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dards, such as BDPA [2,3], DPPH [3], TEMPO [4] or TEM40
POL [5] free radicals. Very recently the ESN-STM has been
Up to now, one of the most promising techniques to detect
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used to complete the characterization of self-assembled monoone single spin carrier molecule deposited on surface is the
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layers of nitronyl nitroxide radicals [6], especially engineered
Electron Spin Noise Scanning Tunnelling Microscopy (ESN43
for self-assembling and gold surface grafting through covalent
STM) technique that combines the spatial resolution of the
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bonding.
STM with the spectral resolution of the Electron Spin Reso45
We report here on the use of the ESN-STM technique to adnance [1]. After its first application to detect defects in oxi46
dress the behaviour of polychlorotriphenylmethyl (PTM) raddized silicon surfaces [1], the effectiveness of this technique
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ical derivatives at a single molecule level. These molecules are
has been tested by various groups who focused their attention
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persistent radicals that find application in nanotechnology as
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molecular wires, switches and memory devices [7]. PTM mol50
ecules can be variously functionalized and moreover present
Q1 * Corresponding author. Tel.: þ34 935 801 853; fax: þ34 935 805 729.
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E-mail addresses: [email protected] (D. Gatteschi), manassen@bgu.
electroactive properties that allow the oneoff switching of
ac.il (Y. Manassen), [email protected] (J. Veciana).
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their
magnetic nature [8]. In addition, PTM can be used as
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Tel.: þ34 935 801 853; fax: þ34 935 805 729.
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building
block for organo-metallic complexes [9] and used
Tel./fax: þ39 055 457 3327.
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for the synthesis of biradical species presenting interesting
Tel.: þ972 08 647 2153; fax: þ972 08 647 2904.
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1293-2558/$ - see front matter Ó 2007 Elsevier Masson SAS. All rights reserved.
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doi:10.1016/j.solidstatesciences.2007.09.021
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Towards the detection of single polychlorotriphenylmethyl radical
derivatives by means of Electron Spin Noise STM
Please cite this article in press as: V. Mugnaini et al., Solid State Sci. (2007), doi:10.1016/j.solidstatesciences.2007.09.021
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2. STM characterization
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By ESR, we demonstrated that the TTM derivative radicals
preserve their magnetism when deposited on the surface,
which is a fundamental requirement for performing ESNSTM experiments.
The grafting of the radical molecules on Au(111) was performed with the dip and rinse procedure, in the same way as
for the STM characterization, and inserted inside a quartz
EPR tube. Spectra were taken at room temperature on the sample freshly prepared as well as after a few days. These measurements unambiguously show that not only these radicals
preserve their magnetism on surface, but they are also stable
in time even when physisorbed on Au(111) surface. The spectrum (Fig. 3) shows only one line as a result of the immobilization of the molecule on the solid surface.
4. ESN-STM characterization
The main concept of ESN-STM is that a spin, in the pres- Q2
ence of an external magnetic field, starts precessing and hence
generates a modulation in the tunnelling current at its Larmor
frequency [1]. Unlike ESR, in this technique no external rf
stimulus is used, and instead, the noise created by the spin
at the Larmor frequency is detected. As known, the Larmor
frequency (nL) is directly related to the g-value of a paramagnetic centre by the relation: nL ¼ gmBB/h. On this basis, for
a fixed external applied magnetic field, it is possible to identify
a paramagnetic species just by looking at the AC component
of the tunnelling current at the frequency value at which resonance occurs. The radio frequency (rf) modulation generated
by the spin precession sums up to the direct tunnelling current
(DC) and by its revelation it is possible to determine the location of the spin site on the surface [12]. By following this rf
component of the tunnelling current flowing through the
STM tip, we can get local information about the paramagnetic
centres located on the surface: the high frequency component
of the tunnelling current is decoupled by the continuous one e
which is fed into a low-pass band IeV converter and used as
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The TTM physisorbed Au(111) samples for STM were prepared using the dip and rinse procedure [3]. Dichloromethane
solution (1 ml, 103 M) of the radical under study was freshly
prepared before every set of measurements. In it, a small piece
(9 mm2) of 150 nm gold evaporated over mica (Molecular
Imaging e Agilent Inc.), previously hydrogen flame annealed
and washed in ultra pure ethanol and dichloromethane, was
immersed for 30 s. Then the same gold was rinsed in ultra
pure dichloromethane to wash out the molecules that were
not physisorbed and gently dried under nitrogen flux. On such
prepared gold samples, well-resolved STM images were obtained working in nitrogen environment and using tunnelling
currents of 15 pA. STM measurements were performed with
a P47-Pro system (NT-MDT, Zelenograd, Moscow, Russia)
equipped with a customized low-current STM head and Pt/Ir
90/10 mechanically-cut tips prepared immediately before use.
As shown in Fig. 2a TTM molecules are adsorbed on the entire
gold surface forming aggregates of a diameter of less than 8 nm.
Four hours after sample preparation, ordered domains were
observed (Fig. 2b). A high resolution image (Fig. 2c) reveals
the detailed structures that are characterized by periodic rows,
distant 1.5 0.1 nm one from the other. The periodicity of
the rows, as well as their dimension, is consistent with the
molecular size of TTM (Fig. 1) [11].
3. CW ESR measurements
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intramolecular electron transfer phenomena [10]. Making use
of the ESN-STM set up already tested on DPPH and BDPA
[3], we investigated the tris(2,4,6-thrichlorophenyl)methyl
radical (TTM) derivative depicted in Fig. 1.
This radical appeared to be an excellent candidate for the
ESN-STM measurements since we proved, with preliminary
STM and CW ESR characterizations here reported, that it fulfils
the requirements of this technique, i.e. (a) ability to selforganize on surfaces forming either small aggregates (nanodots) or well-spaced regular monolayer organizations with
significant molecular separations and (b) preservation of its
magnetism when dispersed on such surfaces.
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Fig. 1. Molecular structure of the TTM radical and its crystal structure representation with indication of the distances between the para-chlorine atoms [12].
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examined and the real signals; this is accomplished by applying an AC magnetic field so as to modulate the Larmor frequency with a modulation frequency to be fed as a reference
in a lock-in amplifier. The proper ratio between the modulation intensity to be chosen and the modulation frequency is
2 and a sketch of the complete apparatus used for all the measurements reported here can be found in Ref. [3]. While recording the STM image, we localized the tip on the TTM
aggregates and recorded an rf spectrum in the frequency range
around the Larmor frequency; during this spectroscopy operation, the tunnelling current was temporarily raised to 1 nA.
The parameter chosen as statistical indicator of the intensity of the candidate signals is the difference of the maximum
positive peak and the minimum negative peak divided by the
standard deviation (Pk Pk/s). For each of the spectra, the
Pk Pk/s parameter was determined using an automatic routine especially written to analyze this type of data [14]. Nevertheless, each of the spectra has been individually checked
to control the possible presence of a proper derivative shape
as well as that the distance between the maximum and the
minimum peak was of some kHz, i.e. comparable with the
concerned parameters of the measurement (the corresponding
modulation intensity of the frequency and the resolution band
width of the spectrum analyzer). Only after this check we
could finally consider the candidate signal as an ESN signal,
as it is the case for the spectrum reported in Fig. 4.
Even though the set up has been designed and realized to be
free of any kind of environmental noise in the radio frequency
range [3], it is important to distinguish between ESN-STM
signal and the environmental noise at the same working frequency. Therefore, we first took over 1000 spectra on the
gold substrate with the tip slightly retracted and out of tunnelling. We could in this way establish the threshold value corresponding to the radio frequency noise level. Once the
threshold was established, we worked in tunnelling conditions
on the radical molecules. The Pk Pk/s histogram for each set
of measurements refers to a set of experiments performed at
one working frequency and with a span of 3 MHz. Due to
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a feedback to perform standard STM operations e so to be
amplified and then monitored by a spectrum analyzer.
An important aspect to deal with when performing these
experiments is the transient nature of the signal, i.e. the intrinsic instability in frequency that may cause failure in detecting
the paramagnetic centres. Therefore, it appears mandatory to
repeat the experiment on many single molecules or aggregates
to get statistically significant results. Moreover, the level of the
signal we want to detect is quite low [3], and only a large number of spectra and a statistical analysis of the collected data
can make these measurements reliable. The phase sensitive detection (PSD) method, implemented by one of the authors
[13], has been used in order to distinguish between spurious
signals which might accidentally exist at the frequency
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Fig. 2. (a) STM image (322 322 nm) of TTM dispersed on Au(111). Tip sample bias 0.3 V and tunnelling current 15 pA. (b) STM image (60 60 nm) of TTM
dispersed on Au(111). Tip sample bias 0.3 V and tunnelling current 15 pA. (c) Zoom of one of the ordered domains: the periodicity of the rows corresponds to
1.5 0.1 nm as shown in the inset figure.
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Fig. 3. Top: the signal of the dip and rinse TTM sample. MW frequency
9.40388 GHz, power 20 mW, Mod. Amp. 1 G, field modulation frequency
100 kHz. g-Value 2.00343; ESR cavity: standard TE102. The over-modulated
spectrum shows one line arising from TTM of Lw ¼ 3.14 G. The small signal
at lower g-value is arising from the Au/mica substrate, as shown in the bottom
spectrum. Bottom: the ESR spectrum of the substrate under the same experimental conditions.
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the uncertainty in the value of the applied magnetic field, experiments were performed in different frequency ranges. The
statistical analysis of all the recorded data shows some peculiar characteristics of the TTM radical with respect to the
noise, opening the way towards the use of ESN-STM to detect
single spins on the surface. While the noise distribution is
symmetric around the central value and fitted properly by
Gaussian symmetric function (Fig. 5), the histogram for
TTM aggregates is right skewed and well fitted by the asymmetric Gaussian function [15]:
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y ¼ y0 þ B exp ðx xc Þ 2s21 and B
¼ A=ðs1 þ s2 =2Þ ð2pÞ0:5 ; if x xc
ð1Þ
y ¼ y0 þ B exp ðx xc Þ 2s22 and B
0:5
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¼ A=ðs1 þ s2 =2Þ ð2pÞ ; if x > xc
ð2Þ
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where the centre of the distribution (xc) and the widths s1 and Q5
s2 are free parameters optimized during the fitting. When using this function to fit the noise data, s1 and s2 turned out to be
coincident, as expected for a symmetric distribution. Instead,
the best fitting of the TTM histogram gave a slightly higher
value for the centre and, which is more interesting, a difference
between s1 and s2 of 0.21, a clear indication of the asymmetry
in the distribution. This asymmetry means that the total number of events whose Pk Pk/s is larger than the xc value of the
noise histogram is higher for the TTM than for the noise.
Therefore, we can conclude that not only the percentage of
spectra above the threshold, but also considerations on the
asymmetry of the statistical distributions must be taken into
account when performing ESN-STM experiments on molecules deposited onto a solid surface. We are currently working
on the elaboration of a physical model able to explain the
asymmetry in our results.
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Fig. 4. Top: ESN-STM image of TTM dispersed on gold, without any filtering.
The sample was prepared by the dip and rinse procedure. While scanning ESN
spectra are acquired in correspondence of detected objects, in particular the ESN
spectrum presented in the bottom part corresponds to the aggregate number 10.
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Fig. 5. Left: the histogram of the out of tunnelling noise (1024 spectra) at 255 MHz and its Gaussian fit. Right: the histogram of the TTM radicals (532 spectra) at
the same frequency and in black its asymmetric fit. As a comparison the fit of the noise (in grey) is superimposed to the TTM one.
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References
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[1] Y. Manassen, R.J. Hamers, J.E. Demuth, A.J. Castellano Jr., Phys. Rev.
Lett. 62 (1989) 2531.
[2] C. Durkan, M.E. Welland, Appl. Phys. Lett. 80 (2002) 458.
[3] P. Messina, M. Mannini, A. Caneschi, D. Gatteschi, L. Sorace,
P. Sigalotti, C. Sandrin, S. Prato, P. Pittana, Y. Manassen, J. Appl.
Phys. 101 (2007) 053916.
[4] C. Durkan, Contemp. Phys. 45 (2004) 1.
[5] P. Krukowski, W. Olejniczak, Z. Klusek, M. Puchalski, P. Kobierski,
K. Gwozdzinski, Appl. Phys. A, submitted for publication.
[6] (a) M. Mannini, L. Sorace, L. Gorini, F.M. Piras, A. Caneschi,
A. Magnani, S. Menichetti, D. Gatteschi, Langmuir 23 (2007) 2389;
(b) M. Mannini, P. Messina, L. Sorace, L. Gorini, M. Fabrizioli,
A. Caneschi, Y. Manassen, P. Sigalotti, P. Pittana, D. Gatteschi, Inorg.
Chim. Acta (2007). doi:10.1016/j.ica.2007.02.005.
[7] C. Sporer, I. Ratera, D. Ruiz-Molina, Y. Zhao, J. Vidal-Gancedo,
K. Wurst, P. Jaitner, K. Clays, A. Persoons, C. Rovira, J. Veciana, Angew.
Chem. Int. Ed. 43 (2004) 5266.
[8] N. Crivillers, M. Mas-Torrent, S. Perruchas, N. Roques, J. Vidal-Gancedo,
J. Veciana, C. Rovira, L. Basabe-Desmonts, B. Jan Ravoo, M. CregoCalama, D.N. Reinhoudt, Angew. Chem. Int. Ed. 46 (2007) 2215.
[9] D. Maspoch, D. Ruiz-Molina, K. Wurst, N. Domingo, M. Cavallini,
F. Biscarini, J. Tejada, C. Rovira, J. Veciana, Nat. Mater. 2 (2003) 190.
[10] V. Lloveras, J. Vidal-Gancedo, D. Ruiz-Molina, T.M. Figueira-Duarte,
J.-F. Nierengarten, J. Veciana, C. Rovira, Faraday Discuss. 131 (2006)
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[13] Y. Manassen, J. Magn. Reson. 126 (1997) 133.
[14] We acknowledge P. Pittana for writing the script to perform the automatic
analysis of the spectra.
[15] The function used is reported in the User Manual of the program Origin,
v. 7.5, CopyrightÓ 2003 by OriginLab Corporation.
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We have demonstrated the effectiveness of the ESN-STM
in locating and studying TTM radical derivatives physisorbed
as small aggregates lower than 10 nm in diameter on gold surface. The obtained promising results prompt us in improving
the control of the dimension of the radicals deposited on
gold and then exploit the possibilities of this technique following how the spin noise signal depends on the nanometer size of
the aggregate. In conclusion, these molecules retain their paramagnetic character, as shown by ESR, which allowed us to
detect a few good ESN spectra. The performed statistical analQ6 ysis makes us confident about our results. The preliminary
results so far obtained look promising, although a proper implementation of the equipment is still required to exploit the
possibilities of this new technique. Further improvements of
the experimental system are in progress. These include improving the matching between the detector and the STM, adding a reactive component in parallel to the STM tunnelling
junction (tuning) and cooling to 4 K. We expect that these
steps will significantly improve our ability to study the paramagnetic molecules at the single spin level.
V.M. and I.R. are grateful to CSIC and Ministerio Educación y
Ciencia for an I3P postdoctoral contract and a ‘‘Ramon y Cajal’’ contract, respectively. Y.M. thanks the WOLFSON and
the BSF foundations for funding.
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5. Conclusions
Acknowledgements
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P. Messina and P. Pittana are gratefully acknowledged for
useful discussions. APE Research Company is acknowledged
for their technical support for ESN-STM instrumentation.
P. Pittana is thanked for the implementation in the automatic
routine analysis of the ESN spectra. This work was supported
by the Dirección General de Investigación (Spain) (Project
Consolider-C EMOCIONa CTQ2006-06333/BQU), the Generalitat de Catalunya (Project 2005 SGR-00591), Italian
MIUR through FIRB and PRIN projects, and European Commission through Network of Excellence MAGMANet (NMP3CT-2005-515767), and QueMolNa (MRTN-CT-2003-504880).
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