Molecules that mimic Schottky diodes

Self-assembled monolayers of cationic donor-(pi-bridge)-acceptor dyes coupled with anionic donors exhibit asymmetric current-voltage (I-V) characteristics when contacted by Au or PtIr probes. Rectification ratios of 3000 at +/- 1 V are obtained from Au-S-C10H20-A+-pi-D|D-|Au structures in which the cationic moiety is 5-(4-dimethylaminobenzylidene)-5,6,7,8-tetrahydro-isoquinolinium and the counterion is copper phthalocyanine-3,4',4',4'-tetrasulfonate (SAM ). Similar behaviour, with a high rectification ratio of 700-900 at +/- 1 V, is also obtained for the CuPc(SO3-)4 salt of 4-[2-(4-dimethylaminonaphthalen-1-yl)-vinyl]-quinolinium (SAM ). The properties are dependent upon the D-pi-A+ moieties which, for these highly rectifying salts, have sterically locked non-planar structures causing the conjugation to be effectively broken. Its effect on the electrical asymmetry is less spectacular when the cationic species is sterically unhindered: the rectification ratio decreases to 15-70 at +/- 1 V for films of the 4-[2-(4-dimethylaminophenyl)-vinyl]-pyridinium salt (SAM ), which has single-ring substituents on opposite sides of the -CH=CH- bridge and an almost planar D-pi-A+ structure. Rectification ratios from the sterically hindered structures are on a par with electrical asymmetries from metal-insulator-metal (MIM) devices where oxide-induced Schottky barriers dominate the behaviour.     

Improved molecular rectification from self-assembled monolayers of a sterically hindered dye

Self-assembled monolayers (SAMs) formed from the reaction of 1-(10-acetylsulfanyldecyl)-4-[2-(4-dimethylaminonaphthalen-1-yl)-vinyl]-quinolinium iodide (1a) and gold-coated substrates exhibit asymmetric current-voltage (I-V) characteristics with a rectification ratio of 50-150 at +/-1 V. It is the highest to date for a molecular diode, and the improved behavior may be assigned in part to the controlled alignment of the donor-(pi-bridge)-acceptor moieties and in part to steric hindrance, which imposes a nonplanar structure and effectively isolates the molecular orbitals of the donor and acceptor end groups. The molecular origin of the rectification is verified by its suppression upon exposure to HCl vapor, which protonates the dimethylamino group and inhibits the electron-donating properties, with restoration upon exposure to NH3. It is also established by a reduced rectification ratio of ca. 2 at +/-1 V when the cationic D-pi-A+ moieties adopt an antiparallel arrangement in self-assembled films of the derivative, bis-[1-(10-decyl)-4-[2-(4-dimethylaminonaphthalen-1-yl)-vinyl]-quinolinium]-disulfide diiodide (1b), which adsorbs via one of its terminal donors without rupture of the sulfur-sulfur bond: Au/D-pi-A+-C10H20-S-S-C10H20-+A-pi-D (I-)2.     

Fabrication and characterization of metal-molecule-metal junctions by conducting probe atomic force microscopy

Metal-molecule-metal junctions were fabricated by contacting Au-supported alkyl or benzyl thiol self-assembled monolayers (SAMs) with an Au-coated atomic force microscope (AFM) tip. The tip-SAM microcontact is approximately 15 nm(2), meaning the junction contains approximately 75 molecules. Current-voltage (I-V) characteristics of these junctions were probed as a function of SAM thickness and load applied to the microcontact. The measurements showed: (1) the I-V traces were linear over +/-0.3 V, (2) the junction resistance increased exponentially with alkyl chain length, (3) the junction resistance decreased with increasing load and showed two distinct power law scaling regimes, (4) resistances were a factor of 10 lower for junctions based on benzyl thiol SAMs compared to hexyl thiol SAMs having the same thickness, and (5) the junctions sustained fields up to 2 x 10(7) V/cm before breakdown. I-V characteristics determined for bilayer junctions involving alkane thiol-coated tips in contact with alkane thiol SAMs on Au also showed linear I-Vs over +/-0.3 V and the same exponential dependence on thickness. The I-V behavior and the exponential dependence of resistance on alkyl chain length are consistent with coherent, nonresonant electron tunneling across the SAM. The calculated conductance decay constant (beta) is 1.2 per methylene unit ( approximately 1.1 A(-)(1)) for both monolayer and bilayer junctions, in keeping with previous scanning tunneling microscope and electrochemical measurements of electron transfer through SAMs. These measurements show that conducting probe-AFM is a reliable method for fundamental studies of electron transfer through small numbers of molecules. The ability to vary the load on the microcontact is a unique characteristic of these junctions and opens opportunities for exploring electron transfer as a function of molecular deformation.     

A molecular half-wave rectifier

This paper describes the performance of junctions based on self-assembled monolayers (SAMs) as the functional element of a half-wave rectifier (a simple circuit that converts, or rectifies, an alternating current (AC) signal to a direct current (DC) signal). Junctions with SAMs of 11-(ferrocenyl)-1-undecanethiol or 11-(biferrocenyl)-1-undecanethiol on ultraflat, template-stripped Ag (Ag(TS)) bottom electrodes, and contacted by top electrodes of eutectic indium-gallium (EGaIn), rectified AC signals, while similar junctions based on SAMs of 1-undecanethiol-SAMs lacking the ferrocenyl terminal group-did not. SAMs in these AC circuits (operating at 50 Hz) remain stable over a larger window of applied bias than in DC circuits. AC measurements, therefore, can investigate charge transport in SAM-based junctions at magnitudes of bias inaccessible to DC measurements. For junctions with SAMs of alkanethiols, combining the results from AC and DC measurements identifies two regimes of bias with different mechanisms of charge transport: (i) low bias (|V| < 1.3 V), at which direct tunneling dominates, and (ii) high bias (|V| > 1.3 V), at which Fowler-Nordheim (FN) tunneling dominates. For junctions with SAMs terminated by Fc moieties, the transition to FN tunneling occurs at |V| ≈ 2.0 V. Furthermore, at sufficient forward bias (V > 0.5 V), hopping makes a significant contribution to charge transport and occurs in series with direct tunneling (V ? 2.0 V) until FN tunneling activates (V ? 2.0 V). Thus, for Fc-terminated SAMs at forward bias, three regimes are apparent: (i) direct tunneling (V = 0-0.5 V), (ii) hopping plus direct tunneling (V ≈ 0.5-2.0 V), and (iii) FN tunneling (V ? 2.0 V). Since hopping does not occur at reverse bias, only two regimes are present over the measured range of reverse bias. This difference in the mechanisms of charge transport at forward and reverse bias for junctions with Fc moieties resulted in large rectification ratios (R > 100) and enabled half-wave rectification.     

Organic rectifying junctions fabricated by ionic coupling

Ionically-assembled structures that comprise discrete layers of cationic acceptors (4,4'-bipyridinium) and anionic donors (copper phthalocyanine-3,4',4',4'-tetrasulfonate) exhibit asymmetric current-voltage (I-V) characteristics with high rectification ratios of 60-100 at +/-1 V.     

Organic rectifying junctions from an electron-accepting molecular wire and an electron-donating phthalocyanine

Self-assembled monolayers (SAMs) of arylene-ethynylene oligomers that incorporate electron-accepting 9-fluorenone and 9-[(4-pyridyl)methylene]fluorene units in the backbone exhibit symmetrical current-voltage (I-V) characteristics, but rectifying junctions with current ratios of 20-80 at +/-1 V have been obtained by protonating these wire-like molecules and ionically coupling with anionic donors.     

Molecular diodes and ultra-thin organic rectifying junctions: Au-S-CnH2n-Q3CNQ and TCNQ derivatives

Attempts to obtain derivatives of the molecular diode, 2-{4-[1-cyano-2-(1-(omega-acetylsulfanylalkyl)-1H-quinolin-4-ylidene)-ethylidene]-cyclohexa-2,5-dienylidene}-malonitrile [1, CH(3)CO-S-C(n)H(2n)-Q3CNQ], from either 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-p-quinodimethane (TCNQF(4)) or 2,3,5,6-tetramethyl-7,7,8,8-tetracyano-p-quinodimethane (TMTCNQ) result in ring closure via the cyano group of the pi-bridge and yield di-substituted analogues: 2-{2,3,5,6-tetrafluoro-4-[6-(10-acetylsulfanyldecyl)-3-(1-(10-acetylsulfanyldecyl)-1H-quinolin-4-ylidenemethyl)-6H-benzo[f][1,7]naphthyridin-2-ylidene]-cyclohexa-2,5-dienylidene}-malonitrile (2a) and the 2,3,5,6-tetramethyl derivative (2b). Self-assembled monolayers (SAMs) of these donor-(pi-bridge)-acceptor molecular diodes exhibit asymmetric current-voltage characteristics with electron flow at forward bias from the top contact to surface C(CN)(2) groups. Comparison is made with I-V curves from ultra-thin films of an organic rectifying junction in which TCNQ(-) is electron-donating and a donor-(sigma-bridge)-acceptor diode in which TCNQ degrees is electron-accepting.     

Chemical force microscopy investigation of phosphate adsorption on the surfaces of iron(III) oxyhydroxide particles

Phosphate-modified AFM tips were prepared by the deposition of self-assembled monolayers (SAMs) of bis(11-thioundecyl) phosphate on Au-coated silicon nitride cantilevers. The properties of the PO(2)H-terminated SAMs were investigated by studying the pH-dependent force interactions of the tips with phosphate- and carboxylic acid-terminated SAM control surfaces. The PO(2)H functional groups had a pK(a) of approximately 5.0. A chemical force microscopy (CFM) study was conducted on the interactions between the probes and the surfaces of hydrous ferric oxide particles prepared in our laboratory by hydrolytic precipitation from FeCl(3). The forces between PO(2)H probes and the hydrous ferric oxide surfaces were seen to exhibit a strong pH dependence, with maximum attractive forces occurring for pH values between 5 and 8. The effects of postprecipitation of the hydrous ferric oxide colloids with orthophosphate, H(2)PO(4)(-), dimethylphosphate, (CH(3)O)(2)PO(2)H (DMP), and tannic acid (TA) on the adhesive interactions between the PO(2)H tips and the solids were also investigated. Attenuated total reflectance infrared spectroscopy (ATR-IR) was used to verify the presence of surface-adsorbed species and zeta potentiometric measurements to determine surface charge on the colloids. We show that the method of chemical force titration using phosphate-terminated tips can differentiate between these various colloids and that it shows promise as a general method for studying this environmentally important class of compounds.     

Length-dependent transport in molecular junctions based on SAMs of alkanethiols and alkanedithiols: effect of metal work function and applied bias on tunneling efficiency and contact resistance

Nanoscopic tunnel junctions were formed by contacting Au-, Pt-, or Ag-coated atomic force microscopy (AFM) tips to self-assembled monolayers (SAMs) of alkanethiol or alkanedithiol molecules on polycrystalline Au, Pt, or Ag substrates. Current-voltage traces exhibited sigmoidal behavior and an exponential attenuation with molecular length, characteristic of nonresonant tunneling. The length-dependent decay parameter, beta, was found to be approximately 1.1 per carbon atom (C(-1)) or 0.88 A(-)(1) and was independent of applied bias (over a voltage range of +/-1.5 V) and electrode work function. In contrast, the contact resistance, R(0), extrapolated from resistance versus molecular length plots showed a notable decrease with both applied bias and increasing electrode work function. The doubly bound alkanedithiol junctions were observed to have a contact resistance approximately 1 to 2 orders of magnitude lower than the singly bound alkanethiol junctions. However, both alkanethiol and dithiol junctions exhibited the same length dependence (beta value). The resistance versus length data were also used to calculate transmission values for each type of contact (e.g., Au-S-C, Au/CH(3), etc.) and the transmission per C-C bond (T(C)(-)()(C)).     

Direct detection by atomic force microscopy of single bond forces associated with the rupture of discrete charge-transfer complexes

Atomic force microscopy (AFM) was used to measure the chemical binding force of discrete electron donor-acceptor complexes formed at the interface between proximal self-assembled monolayers (SAMs). Derivatives of the well-known electron donor N,N,N',N'-tetramethylphenylenediamine (TMPD) and the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) were immobilized on Au-coated AFM tips and substrates by formation of SAMs of N,N,N'-trimethyl-N'-(10-thiodecyl)-1,4-phenylenediamine (I) and bis(10-(2-((2,5-cyclohexadiene-1,4-diylidene)dimalonitrile))decyl) disulfide (II), respectively. Pull-off forces between modified tips and substrates were measured under CHCl(3) solvent. The mean pull-off forces associated with TMPD/TCNQ microcontacts were more than an order of magnitude larger than the pull-off forces for TMPD/TMPD and TCNQ/TCNQ microcontacts, consistent with the presence of specific charge-transfer interactions between proximal TMPD donors and TCNQ acceptors. Furthermore, histograms of pull-off forces for TMPD/TCNQ contacts displayed 70 +/- 15 pN periodicity, assigned to the rupture of individual TMPD-TCNQ donor-acceptor (charge-transfer) complexes. Both the mean pull-off force and the 70 pN force quantum compare favorably with a contact mechanics model that incorporates the effects of discrete chemical bonds, solvent surface tensions, and random contact area variations in consecutive pull-offs. From the 70 pN force quantum, we estimate the single bond energy to be approximately 4-5 kJ/mol, in reasonable agreement with thermodynamic data. These experiments establish that binding forces due to discrete chemical bonds can be detected directly in AFM pull-off measurements employing SAM modified probes and substrates. Because SAMs can be prepared with a wide range of exposed functional groups, pull-off measurements between SAM-coated tips and substrates may provide a general strategy for directly measuring binding forces associated with a variety of simple, discrete chemical bonds, e.g., single hydrogen bonds.     

First Principle Study on the Rectification of Molecular Junctions Based on the Thiol-ended Oligosilane

The electron transport properties of various molecular junctions based on the thiol-ended oligosilane are investigated through density functional theory combined with non-equilibrium Green's function formalism. Our calculations show that oligosilanes doped by the phenyl and-C10H6 groups demonstrate better rectifying effect and their rectification ratios are up to 15.41 and 65.13 for their molecular junctions. The current-voltage(I-V) curves of all the Au/ modified oligosilane/Au systems in this work are illustrated by frontier molecular orbitals, transmission spectra and density of states under zero bias. And their rectifying behaviors are analyzed through transmission spectra.     

Surface microscopic structure and electrochemical rectification of a branched alkanethiol self-assembled monolayer

The tert-butanethiol self-assembled monolayers (SAMs) on Au(111) surfaces were prepared from various solvents and investigated by a combination of scanning tunneling microscopy (STM) and electrochemistry in aqueous environments. High-resolution STM images reveal a (radical(7) x radical(7))R19 degrees surface lattice structure, in contrast with the conventional lattice (radical(3) x radical(3))R30 degrees structure for straight-chain alkanethiol SAMs. Interestingly, such a branched monolayer shows electrochemical rectification toward redox probes. We suggest that electrochemical rectification could be a general characteristic of short-chain branched alkanthiol SAMs, and originate in localized electronic effects.     

Optical and electronic properties of polyaniline sulfonic acid-ribonucleic acid-gold nanobiocomposites

Finely fibrillar polyaniline sulfonic acid (PSA)/ribonucleic acid (RNA) hybrids are developed by wrapping PSA with RNA from a mixture of aqueous PSA (P) and RNA (R) solutions of different compositions. FTIR spectra suggest H-bonding and π-π interactions in the hybrids and dedoping of self doped PSA during hybrid formation. UV-vis spectra exhibit a blue shift of the π-band to polaron band transition of PSA from 870 to 581 nm due to dedoping. The PR hybrids show enhanced PL-properties when excited at 540 nm relative to PSA which also exhibits rectification behavior in current (I)-voltage (V) curves. Gold nanoparticles (Au NPs) grown on these PR hybrids by the reduction of Au(3+) by PSA show different morphologies with varying composition. FTIR spectra of the nanobiocomposites indicate that Au NPs are stabilized by the co-ordination of the nitrogen atoms of -N=Q=N- bonds of PSA (Q = quinonoid ring). The intensity of the Au plasmon band gradually decreases with time but the PL-intensities of the PAu/PRAu nanocomposites increase with time. The PL-intensity of the nanocomposites is higher than that of PSA and PR hybrids. The DC-conductivity of the PR hybrids increases by an order of magnitude on addition of Au NPs. I-V curves of the nanobiocomposites show negative differential resistance (NDR) in PSA rich systems with a stable NDR ratio of 7 in the PRAu21 and PRAu11 hybrids. Possible reasons from the accumulation of charges on the Au NPs and its stabilization through the π-clouds of RNA bases are discussed. The PRAu11 system also exhibits rectification properties with a rectification ratio of 14.     

In situ spectroscopic characterization of rectifying molecular monolayers self-assembled on gold.

We report visible, Raman, and infrared spectra of self-assembled monolayers (SAMs) formed by the donor-(pi-bridge)-acceptor chromophore, Z-beta-[N-(omega-acetylthioalkyl)-4-quinolinium]-alpha-cyano-4-styryldicyanomethanide (CH3CO-S-CnH2n-Q3CNQ where n=8, 10), on gold-coated substrates. The data are compared with the spectra collected for the same compound in solution and in the solid state, and with those obtained for a Langmuir-Blodgett (LB) monolayer of C16H33-Q3CNQ deposited on gold. Spectral analysis confirms that in solution, in the solid state and in the LB film the chromophore has a zwitterionic (D+-pi-A-) ground state. At variance with this well-known result, our data show that in SAMs deposited on gold the chromophore has a more neutral, quinoid ground state. We relate this difference to the different packing of the molecules in the two different films: in SAMs in fact the chromophores stand almost vertical with respect to the substrate, whereas in LB films they make an angle of about 45 degrees. The Q3CNQ molecule is a well-known molecular rectifier, and for SAMs we were able to check the direction of electron flow at forward bias on the same samples that have been characterized spectroscopically, shedding light on the rectification mechanism.     

Combined STM and FTIR characterization of terphenylalkanethiol monolayers on Au(111): effect of alkyl chain length and deposition temperature

Self-assembled monolayers (SAMs) of 4,4'-terphenyl-substituted alkanethiols C6H5(C6H4)2(CH2)n-SH (TPn, n = 1-6) on Au (111) substrates were studied using scanning tunneling microscopy (STM) and infrared reflection absorption spectroscopy (IRRAS). When the SAMs were prepared at room temperature (RT, 298 K), TPn films (except TP2) exhibit an odd-even effect regarding both molecular orientation and packing density. For all investigated films, STM data reveals the presence of a large degree of lateral order. In the case of odd-numbered TPns, the films revealed a (2 square root(3) x square root(3))R30 degree molecular arrangement. For the even-numbered TP4 and TP6 SAMs, a c(5 square root(3) x 3) rectangular unit cell was found. The packing density for the even-numbered TPn SAMs is 25% lower than that for the odd-numbered TPn SAMs. When the SAMs were prepared at 333 K, the even-numbered SAMs were found to form structures with a significantly lower packing density. In the case of TP2, instead of the (2 square root(3) x square root(3))R30 degree structure formed at room temperature, a c(5 square root(3) x 3) structure was observed. For TP6 SAMs, the room-temperature c(5 square root(3) x 3) structure was replaced by a (6 square root(3) x 2 square root(3))R30 degree structure.     

Mechanism of rectification in tunneling junctions based on molecules with asymmetric potential drops

This paper proposes a mechanism for the rectification of current by self-assembled monolayers (SAMs) of alkanethiolates with Fc head groups (SC(11)Fc) in SAM-based tunneling junctions with ultra-flat Ag bottom electrodes and liquid metal (Ga(2)O(3)/EGaIn) top electrodes. A systematic physical-organic study based on statistically large numbers of data (N = 300-1000) reached the conclusion that only one energetically accessible molecular orbital (the HOMO of the Fc) is necessary to obtain large rectification ratios R ≈ 1.0 × 10(2) (R = |J(-V)|/|J(V)| at ±1 V). Values of R are log-normally distributed, with a log-standard deviation of 3.0. The HOMO level has to be positioned spatially asymmetrically inside the junctions (in these experiments, in contact with the Ga(2)O(3)/EGaIn top electrode, and separated from the Ag electrode by the SC(11) moiety) and energetically below the Fermi levels of both electrodes to achieve rectification. The HOMO follows the potential of the Fermi level of the Ga(2)O(3)/EGaIn electrode; it overlaps energetically with both Fermi levels of the electrodes only in one direction of bias.     

Selective anion sensing based on tetra-amide calix[6]arene derivatives in solution and immobilized on gold surfaces via self-assembled monolayers

Two anion receptors, 1 and 2, based on the calix[6]crown-4 architecture were synthesized and characterized by NMR (1H, 13C, COSY), UV-vis, and MALDI-MS. 1H NMR measurements demonstrate that receptors 1 and 2 exhibit the highest binding affinity for fluoride ions compared to other anions including Cl-, Br-, NO3-, HSO4-, H2PO4-, and AcO-. The binding constants of 1 with F- and AcO- are 326 (+/-32) and 238 (+/-23) M-1, whereas those of 2 with F- and AcO- are 222 (+/-25) and 176 (+/-21) M-1. The fluorescent titration of 2 with various anions such as Cl-, Br-, NO3-, HSO4-, and H2PO4- led to essentially no change in excimer emission and a slight enhancement of monomer emission. In contrast, a dramatic change was observed in the fluorescence spectra upon the addition of F- and AcO- to 2. Self-assembled monolayers (SAMs) of 1 were formed on gold surfaces and characterized by reductive desorption and other techniques. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy were used to monitor anion recognition by the SAM-modified gold electrodes. The gold electrodes modified by SAMs of 1, upon binding with the F- anion, exhibit a dramatic increase in charge-transfer resistance (Rct) values. This is due to the repulsion between the negatively charged electrode surfaces and the negatively charged Fe(CN)6(3-/4-) redox probe in the electrolyte solution. In contrast, smaller increases in Rct values were observed in the cases of other monovalent anions investigated.     

Synthesis and properties of functionalized 4 nm scale molecular wires with thiolated termini for self-assembly onto metal surfaces

We report the synthesis of new oligo(aryleneethynylene) molecular wires of ca. 4 nm length scale by palladium-catalyzed Sonogashira cross-coupling methodology. Key structural features are the presence of electron donor 9-(1,3-dithiol-2-ylidene)fluorene (compounds 13 and 14) and electron acceptor 9-[di(4-pyridyl)methylene]fluorene units (compound 16) at the core of the molecules. Terminal thiolate substituents are protected as cyanoethylsulfanyl (13 and 16) or thioacetate derivatives (14). The molecules display well-defined redox processes in solution electrochemical studies. The optical properties in solution are similar to those of the fluorenone analog 6: the strongest absorptions for 6, 13 and 16 are in the region lambda(max) = 387-393 nm, with 13 showing an additional shoulder at 415 nm which is not present for 6 and 16; this shoulder is assigned to a HOMO-LUMO transition from the dithiole to the fluorene unit. Molecules 6, 13, 14 and 16 form self-assembled monolayers on gold substrates which exhibit essentially symmetrical current-voltage (I-V) characteristics when contacted by a gold scanning tunelling microscope (STM) tip. The effects of the chemical modifications at the central unit of 6, 14 and 16 on the HOMO-LUMO levels and electron transport through the molecules in vacuum have been computed by an ab initio approach.     

Catalytic probe lithography: catalyst-functionalized scanning probes as nanopens for nanofabrication on self-assembled monolayers

This article describes the use of scanning catalytic probe lithography for nanofabrication of patterns on self-assembled monolayers (SAMs) of reactive adsorbates. Catalytic writing was carried out by scanning over bis(omega-tert-butyldimethyl-siloxyundecyl)disulfide SAMs using 2-mercapto-5-benzimidazole sulfonic acid-functionalized gold-coated AFM tips. The acidic tips induced local hydrolysis of the silyl ether moieties in the contacted areas, and thus patterned surfaces were created. Diffusion effects arising from the use of an ink were excluded in these type of experiments, and therefore structures with well-defined shapes and sizes were produced. The smallest lines drawn by this technique were about 25 nm wide, corresponding to the actual contact area of the tip. Lateral force microscopy studies performed on different SAMs helped to clarify the nature and cause of the friction contrasts observed by AFM. Dendritic wedges with thiol functions inserted into the catalytically written areas, thus enhancing the height contrast. The created patterns open possibilities to build 3D nanostructures.     

Modification and stability of aromatic self-assembled monolayers upon irradiation with energetic particles

We have studied ion and electron irradiation of self-assembled monolayers (SAMs) of 2-(4'-methyl-biphenyl-4yl)-ethanethiol (BP2, CH3-C6H4C6H4CH2CH2-SH), phenyl mercaptan (PEM, C6H5CH2CH2-SH), and 4'-methyl-biphenyl-4-thiol (BP0, CH3-C6H4C6H4-SH) deposited on Au(111) substrates. Desorption of neutral particles from PEM/Au and BP2/Au was investigated using laser ionization in combination with mass spectrometry. The ion-induced damage of both BP2 and PEM SAMs is very efficient and interaction with a single ion leads to the modification of tens of molecules. This feature is the result of a desorption process caused by a chemical reaction initiated by an ion impact. Both for ions and electrons, experiments indicate that the possibility for scission of the Au-S bond strongly depends on the chemical nature of the SAM system. We attribute the possible origin of this effect to the orientation of the Au-S-C angle or adsorption sites of molecules. The analysis of electron-irradiated PEM/Au and BP2/Au, using ion-initiated laser probing, enabled measurements of the cross section for the electron-induced damage of the intact molecule or specific fragment. Analysis of electron-irradiated BP0/Au by using time-of-flight secondary ion mass spectrometry (TOF-SIMS) provides direct evidence for the quasi-polymerization process induced by electron irradiation.