NANOTECHNOLOGY AND SNIFFING OUT EXPLOSIVESDavid McIlroy, Department of PhysicsDavid McIlroy, Department of PhysicsSupported by the DoD‐AROMOTIVATION¾Explosive detection key for reduced p y fcasualties ¾Early warning: parts per trillion (1011/1023)¾Current methods and methodology insufficient¾ A paradigm shift in our scientific understanding is in order2WHY NANOSPRINGS?¾Can a nanostructure enhance sensitivity?¾Will ‘nano’ help us achieve selectivity (descrimination), and if so, why?¾What chemistry should we use and why does it work? (Linchpin of the study)¾In terms of sensing, what differentiates on type of explosive from another if their fd tl bildig bl k th ?fundamental building blocks are the same?3BA KengneTHE SCIENTIFIC QUESTIONS¾Because they are small they should have d iti itpronounced sensitivity¾A mat of nanosprings is an easy platform to work withwork with¾The surface of nanosprings can be engineered by chemical surface g y ffunctionalization, as well as with surface coatings¾Tbili¾Tunability4Droplet DynamicsDroplet DynamicsDroplet DynamicsThe dynamics ofthe droplet is tedopetsdictated by theinterfacial surfacetension.Contact Angle AnisotropyWk f dh i f dlt t iWk f dh i f dlt t iWk f dh i f dlt t iWork of adhesion of droplet to nanowireWork of adhesion of droplet to nanowireWork of adhesion of droplet to nanowireR:ρ:ΔR:ρ:ΔsphericalcatalystThe Big Break!New Process for SiO2Nanosprings¾ Proposed sensing mechanismTHEORETICAL BACKGROUND(b)BACKGROUND• Nanoscale Schottkycontacts(a)()I‐Schottky contact (Metal/semiconductor) = Non linear behavior (a)V‐Ohmic contact (Metal/Metal) Linear (Metal/Metal) = Linear behavior (b)9Pt Pt on SiOon SiO2222573 K723 K57317 Pa723 K42 PaAvg: 2.5‐3 nm873 Kg 53SA: 95 m2/g7367 PaPRELIMINARY RESULTSO11BA KengneSiO2 NS/ZnO/Ag NP: SEM and TEM images• Significant fraction of atoms occupy surface sites —for Au catalysis not all surface sites are equalsurface sites are equal• Example: C=O groups preferentially activated on {111} surfacesC=C activated at corner and edge sitesFor 3 nm cuboctahedron:3Corner atoms 5% Edge atoms 25%{100} faces 10% {111} faces 60%THEORETICAL OF SCHOTTKY CONTACTSBand bending (Vs)/Modulation of Schottky (Surface) barrier height (Eb= Ø‐χ )EVacEVacØEØχVscEEVacEVacEFcEØVBEMetalVBEMetalEFp‐typeSemiconductorn‐type SemiconductorTHEORETICAL OF SCHOTTKY CONTACTSEEvacEvacØEEcØEcEFEFEcn‐typeMetalMoleculeVBEVBEsn‐typeMetalMODEL OF SENSOR SENSOR BEHAVIORDepletedregionW0-DepletedregionVaWaSensing Mechanismg+++++++++++++++++++---------++++++(a)(b)+++++++++++++++++++---------+++++++++++++++++++++++++---------+++++++++++++++++++++++++---------++++++(a)(b)Nanoscale Schottkycontacts‐Depletion layer‐Gas adsorption0246(mA)0246(mA)0246(mA)‐Gas adsorption‐Change in the depletion depth‐Change in the conductivity -5 -4 -3 -2 -1 0 1 2 3 4 5Vsd (V)-6-4-20Isd VacuumH2COCO+H2O (low)CO+H2O (high)(c) (d)-5 -4 -3 -2 -1 0 1 2 3 4 5Vsd (V)-6-4-20Isd VacuumH2COCO+H2O (low)CO+H2O (high)-5 -4 -3 -2 -1 0 1 2 3 4 5Vsd (V)-6-4-20Isd VacuumH2COCO+H2O (low)CO+H2O (high)(c) (d)of the matNWS = FET, NPs = GatesSource‐drain conductivity modulated by gas modulated by gas adsorptionResponse to toluene at 100C (Resistance vs. Time)ZnO nanosprings with ggold nanoparticlesBare ZnO nanospringsBare ZnO gives higher sensitivityResponse to toluene at 100C (Resistance vs. Time)ZnO nanosprings with SILVER nanoparticlespg pDrops of resistance, instead of the peaks in this case. Arrows indicate Injections of toluene. Possible tunneling transport between the nanoparticles, where conductivity increases because of the additionaldielectric permittivity of vapor.Ag NP coated ZnOPt coated ZnO, Pd coated ZnO, tolueneDNPDNTCysteine treated (Specific to TNT)MOLECULAR FUNCTIONALIZATION FOR SPECIFICITY1.nitro-aromatics of TNT, Picric acis and Tetryl2.military explosives (RDX, HMX , CL-20)2.military explosives (RDX, HMX , CL20)3.nitrate esters (PETN)4.peroxide based compounds (HMTD, TATP)OPTIONS FOR MOLECULAR FUNCTIONALIZATIONEnd group Explosive Interaction mode‐COOH Nitro‐substitutedmoleculesIonic bondsmolecules‐OH, ‐Cl Hydrogen bonds‐CH3 Gaseous explosivesvan der Waals bonds , hd hbi ¾Coatings selective for explosive analytes: functionalized alkanes, hydrophobic effects¾Coatings selective for explosive analytes: functionalized alkanes, polymers, peptides, antibodies¾ Self assembled monolayers (SAMs) of alkyl thiols on Au NPs are betterBind strongly to Au‐Bind strongly to Au‐ Create molecular interaction layers with excellent long term stability¾ Alkyl thiols with different end groups target specific explosive vaporsWHAT’S BEEN TRIEDExplosivesRDX, PETNTNT, DNT TATP TATP , HMTDSAMs 4‐MBA 6‐MNA & hexafluoroisopropanol, peptidesC18‐alkanesmethacrylatepeptides23Sample 1: I‐V characterization of bare ZnO nanosprings in the ambient air, toluene and DNT. Air – yellow (4 curves), toluene –red (7 curves), DNT – green (7 curves). C tbl6.00E-05Curves are repeatable.2.00E-054.00E-05AIR Current_1 (1)AIR Current_1 (1)AIR Current_1 (1)AIR Current_1 (1)Toluene Current_1 (1)0.00E+00-1.50E+00 -1.00E+00 -5.00E-01 0.00E+00 5.00E-01 1.00E+00 1.50E+00Toluene Current_1 (1)Toluene Current_1 (1)Toluene Current_1 (1)Toluene Current_1 (1)Toluene Current_1 (1)Toluene Current 1 (1)-4.00E-05-2.00E-05Toluene Current_1 (1)DNT Current_1 (1)DNT Current_1 (1)DNT Current_1 (1)DNT Current_1 (1)DNT Current_1 (1)-8.00E-05-6.00E-05DNT Current_1 (1)DNT Current_1 (1)8.00E 054.00E-056.00E-05Sl d f b ZO 0.00E+002.00E-05-1.50E+00 -1.00E+00 -5.00E-01 0.00E+00 5.00E-01 1.00E+00 1.50E+00Samples 1, 2, and 3 of bare ZnO nanosprings together. All measurements are repeatable. Change of conductivity in different environments is inconsistent. -6.00E-05-4.00E-05-2.00E-05-8.00E-056.00E-068.00E-064.00E-066.00E-060.00E+002.00E-064.00E-06-1.50E+00 -1.00E+00 -5.00E-01 0.00E+00 5.00E-01 1.00E+00 1.50E+00-2.00E-060.00E+002.00E-06-1.50E+00 -1.00E+00 -5.00E-01 0.00E+00 5.00E-01 1.00E+00 1.50E+00-6.00E-06-4.00E-06-2.00E-068 00E 06-6.00E-06-4.00E-0600 06-8.00E-06-8.00E-06ZnO nanosprings, coated with gold, functionalized with6‐amino‐1‐hexanolAmino 06-30-101.50E-052.00E-05Air Current_1 (1)Air Current_1 (1)Air Current_1 (1)Air Current_1 (1)Air Current_1 (1)-5 00E-060.00E+005.00E-061.00E-05-1.50E+00 -1.00E+00 -5.00E-01 0.00E+00 5.00E-01 1.00E+00 1.50E+00Air Current_1 (1)Air Current_1 (1)DNT Current_1 (1)DNT