SONA : SOurce d’électrons ultra-froids pour NAnofonctionalisation Daniel Comparat Laboratoire Aimé Cotton, CNRS, UPR3321, Bât. 505, Univ Paris-Sud, Orsay, 91405 FRANCE

Equipes et/ou services impliqués : Pour le LAC : Equipe : Physique moléculaire et molécules froides : Gaz de Rydberg gelé et plasma ultra-froid. Daniel Comparat, Chargé de recherche Pierre Pillet, Directeur de recherche Yoann Bruneau + Guyve Khalili, Doctorants   Pour l’ISMO : Equipe SIREN : Surfaces propriétés, réactivités, nanostructuration : Interaction électrons / molécules condensées, électrons / systèmes chimisorbés Anne Lafosse, Professeur Roger Azria, Directeur de recherche Lionel Amiaud, Maîtres de Conférences Justine Houplin, Doctorante

But annoncé Le but du projet est de finaliser une collaboration entre l’équipe du LAC et celle de l’ISMO sur l’utilisation d’une source révolutionnaire d’électrons monocinétiques de basse énergie pour la nanofonctionalisation de molécules sur surface. La source sera développée au LAC à partir d’un plasma ultra-froid obtenu en ionisant des atomes de césium préalablement refroidi par laser. Elle sera ensuite déplacée sur le site de l’ISMO pour y être couplée à une expérience existante. Le but premier sera de confirmer les grandes promesses offertes par cette source à savoir une focalisation, et donc flux d’électrons par molécule, amélioré d’un facteur 1000 par rapport aux sources existantes. Le financement demandé est de 10k€ LAC pour l’achat du laser d’ionisation (770nm). 10k€ ISMO pour la modification de l’enceinte à vide.

Sources d’electrons Technologies actuelles Thermionic (LaB6) Photocathode Cold emission Plasma Limitations: Dispersion d’énergie ∆E=0.3eV Limite la focalisation à faible énergie cinétique du jet: (10µm)x(∆E/E)

e- beam assisted lithography – spatial structuration Development of molecular platforms for chemical or biological devices Response to irradiation (standard probing techniques & lithography) of resist materials (contrast, sensitivity, defects)  2nd electrons contribution Cyganik J.Phys.Chem.B 2005 Heister Langmuir 2000 At high energy (2-20 keV) Electron microscope (EBL) Nanometer scale (20-200 nm) Gölzhäuser Adv. Mat. 2001 Ballav Angew. Chem. 2008 Eck Adv. Mater. 2000 Turchanin Adv. Mater. 2008 At low energy (~50-100 eV) Electron irradiation through masks Micrometer scale (2 mm) Electron induced chemistry is also of interest in the frame of the spatial structurations of substrates, and the preparation of molecular plateforms for developing chemical and biological sensors. One of the methods for substrates structuration is electron beam assisted lithography. A crucial point is then the response of the used resist layer to irradiations, either resulting from conventional characterization procedures by XPS or NEXAFS, or associated to the lithographic procedure itself. It is in particular necessary to characterize the response of the resists to low-energy electrons, in order to improve and optimize its properties like contrast, sensitivity and defect density. I have chosen two examples dealing with electron beam lithography of aromatic Self-Assembled Monolayers (SAMs), more precisely biphenylthiol SAMs. In both cases, electron irradiation was used to stabilize the irradiated zones, by inducing cross-linking between the phenyl rings within the molecular film. When using keV electrons, extremely well focussed beams can be obtained and the spatial resolution can reach 20 nm. When using lower energy electrons, focussed beams can not be easily prepared and the irradiations are performed through masks, whose patterns have typically a micrometer scale resolution.

Point source / colimated source Brightness Br = I/(AWE) : amount of current in a spot Want small A, small W I ~ 1 nA A ~ 100 nm2 E ~ 1-105 eV DE ~1eV Small area A Coulomb explosion Large W, DE ~1eV Ions + Electrons - Conventional sources New: Large area A ~ 1 mm2 No Coulomb explosion Small W, DE ~1 meV - + LARGE SOURCE Needs: Mono-Energy (∆E < 0.1 eV) spectroscopy-chemistry-focus

Ionization of COLD ATOMS ! k B T e G. Freinkman, A. V. Eletskii, and S. I. Zaitsev, Microelectron. Eng. 73, 139 (2004). Laser A 1 K ~ 0.1 meV OTHER ADVANTAGES: Low energy E for less damage Less aberrations (cheaper electrostatic optics)

Proposal for ultracold electron sources Rb Magneto-Optical Trap Values Temperature 1 mK Atom Number 10 9 (/s) Pulsed extraction 1MV/cm in 1ns B > 109 A.rad-2m-2 V-1 Claessens BJ, van der Geer SB, Taban G, Vredenbregt EJ, Luiten OJ. PRL 95, 164801 (2005)  e- diffraction, FEL or accelerator input

Melbourne (R. Scholten) Nature Physics 7 785 (2011).

Example on low energy dispersion Eindhoven (E. Vredenbregt) Rb MOT: electron pulse PRL 105, 034802 (2010)            Eacc ~30µm 1nA                                  differential voltage problem: Coulomb explosion DU < 0.02 eV (ultimate Tion > 10 K) Beam Energy as low as 1 eV

Limitations Low flux well bellow 1nA MOT ~ 107-109 atoms/s  I < 1nA Better to use atomic beams 2D-MOT ~ 109-1011  I < 10nA atomic beams ~ up to 1014  I < 1µA Pulsed behaviour (advantage to play with time dependent fields) Directly ionize atoms (need lots of laser power  pulsed laser) UltraCold Plasma formation NIST PRL 83, 4776 (1999) Better to use Rydberg atoms Our PRL 85 4466 (2000) Needs typically 10 000 lower laser power  CW laser possible Possibility to solve the differential voltage problem ?

Electron source: Coldjet V0 1 Beam Creation 2 Optical Molasses: Beam Collimation 3 2D-Mot: Beam Compression 4 Laser excitation to Rydberg state 5 Field Ionization Extraction 6 To ISMO 1 2 3 4-5 Longitudinale speed(m/s) 300 Divergence (mrad) 27 0.3 Flux (at/s) 1013 1012 Diameter (mm) 8 0.2-1

The electron source - 2D cesium MOT Flux ~ 1010 - 1011 atoms/s => up to 10nA current Flexible setup, to be connected to existing experiments Controlled breaking of molecular bonds (ISMO) Electron Energy Loss Spectroscopy + Microscope (LPS, A. Gloter) Laser setup + Control electronics 2D-MOT Electron optics Host experiment UHV Optical fibers

4. Ionization: excitation to Rydberg state Previous experiments: Near threshold laser ionization Current 1 nA  E > 1kV/cm (waist >10µm)  DV = E*waist > 1 Volt TOO HIGH!!! Our experiment: Field ionization of Rydberg atoms Rydberg atoms field ionized at a given electric field E = 1 /16n4 Ionization at a given position  No differential voltage problem Better excitation efficiency (sexc/sion = 104 for n=30) Choice of the extraction field (n-dependent) with reduction of space charge effects n~30

5. Field Ionization. Electrodes design 18.5 kV -21.4 kV Exctraction electrodes Field ionization area Rydberg excitation lasers Atomic Beam Rydberg excitation in flat electric field  No energy shift Extraction to avoid aberrations

Theoretical study: focusing General Particle Tracer® 10pA e- current 8 eV Beam  DE < 10 meV Focused on ~ (10 nm)2 equivalent to 105 µA on (1mm)2 >103 times that of a standard gun limited space charge effects: DE ~ 16 meV/nA

Expected performances Gun : Tungsten LaB6 Schottky CFEG 2-D MOT Source Brightness (A.m-2 sr-1 eV-1) 104 105 107 108 > 108 Cathode energy spread (eV) 1 0.5 0.4 0.3 < 0.01 T electrons (MOT / beam) < 10K 10 K ~ 1 meV T electrons (Tungsten) ~ 2000 K limited space charge effects: DE ~ 16 meV/nA demonstrated with photoionization of effusive atomic beams (H. Hotop) Review of Scientific Instruments, 72, 4098, (2001).

System(s) proposed for validation Self-Assembled Monolayers (SAMs) – Thiols on gold TPT – Terphenylthiol HS-(C6H4)2-C6H5 / Au Improved stability by irradiation induced cross-linking  Resonant process at 6 eV leading to sp2 → sp3 conversion 11-mercaptoundecanoic acid HS-(CH2)10-COOH -COOH = chemical anchor for DNA and protein immobilization  Resonant process at 1 eV leading –COOH → –C–O–C– 4-mercaptobenzoic acid HS-(C6H4)-COOH I now move on to the second set of results, this one, dealing with the TPT SAM, consisting of aromatic spacers and no terminal function.

Towards layer by layer chemistry – Irradiation strategy Sub-Excitation Region E1 E (eV) (M- ) * (10-12-10-16 s) electron attachment Resonant, efficient, selectivite (functional group / site)  Tuning E(e-) for successive Dissociative Electron Attachement Controlled chemistry by efficient formation of primary reactive species, or by activation of molecular sites Develop low energy electron irradiation strategies leading to controlled layer by layer chemical modifications of the SAM 9eV 1eV Deposited molecular film A dissociative electron atttachement process is a resonant process taking place in a delimited energy window. It was proven to be functional group selective and site selective, and in general one given dissociative channel appears to be quite dominant. So that by tuning the incident electron energy, selective formation of reactive fragments or selective activation of targetted molecular sites can be achieved. The aim of these studies is to developp low-energy electron irradiation strategies leading to controlled layer by layer chemical modifications of the SAM. The aim is to optimize an irradiation sequence targeting successively the spacer layer and the terminal functions. Identify the products formed by low-energy electron induced chemistry Understand the involved mechanisms, in particular to determine the primary interaction mechanism between electrons with molecules Propose selective reaction pathways driven by electron irradiation, taking advantage of the selectivity of the DEA processes Terminal group 6eV Aromatic Alkyl spacer S Au (Arrandee) 19

But annoncé Le but du projet est de finaliser une collaboration entre l’équipe du LAC et celle de l’ISMO sur l’utilisation d’une source révolutionnaire d’électrons monocinétiques de basse énergie pour la nanofonctionalisation de molécules sur surface. La source sera développée au LAC à partir d’un plasma ultra-froid obtenu en ionisant des atomes de césium préalablement refroidi par laser. Elle sera ensuite déplacée sur le site de l’ISMO pour y être couplée à une expérience existante. Le but premier sera de confirmer les grandes promesses offertes par cette source à savoir une focalisation, et donc flux d’électron par molécule, amélioré d’un facteur 1000 par rapport aux sources existantes. Le financement demandé est de 10k€ pour l’achat du laser d’ionisation (770nm) et 10k€ pour la modification de l’enceinte à vide de l’ISMO.

But Réalisé Collaboration LAC-ISMO toujours en cours. * La ionisation de la source est prévue avant fin 2012. * Elle sera ensuite déplacée sur le site de l’ISMO. Financement LUMAT utilisé: 10k€ pour l’achat du laser d’ionisation (770nm). Moglabs 9k€ pour la modification de l’enceinte à vide de l’ISMO (c/ Pfeiffer): pompes à vide (pompage différentiel) composants (brides d’adaptation) 1k€ Diverses optiques (Thorlabs) Pompage : Une turbo, une primaire, une jauge cathode froide

CONCLUSION: High flux of (ordered) ions/electrons By field Rydberg ionization of cold atoms or molecules Laser cooling = collimation Rydberg = excitation + Field ionization 2011 2016 Huge domain of applications and improvement for: Spectrometers, irradiation, induced chemistry Imaging Sputtering, deposition Improve focus limit (10µm)x(∆E/E) Futur: Industrial product? Implantation of N atoms from Cooling of CN molecules