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1 Electroic Supplemetary Material (ESI) for Chemical Commuicatios. This joural is The Royal Society of Chemistry 2016 Supplemetary Iformatio Electrical properties of Flexible Multi-chael Si Naowire Field-Effect Trasistors Depedig o the Number of Si Naowires Do Hoo Kim, Su Jeog Lee, Sag Hoo Lee, Jae-Mi Myoug Departmet of Materials Sciece ad Egieerig, Yosei Uiversity 134 Shicho-dog, Seodaemoo-gu, Seoul, Korea * jmmyoug@yosei.ac.kr Fig. S1. Schematic illustratio showig the formatio of ui-directioal aowires o water surface base o spreadig coefficiet. Whe the dispersio solutio (alcohol) is dropped o a water surface, it quickly spreads o the water surface ad geerates a temporary alcohol-water bilayer. Some fractio of the alcohol evaporates, ad the other is gradually mixed i the water. Oce the alcohol is dropped o water, the NWs i alcohol spread o the water surface alog the alcohol flow. Here, the spreadig coefficiet (S) is defied as follows: S = γ w,a ( γ alc,w + γ alc,a ) = ( γ w,a γ alc,w ) γ alc,a, where γ w,a, γ alc,w, ad γ alc,a are the iterfacial tesios betwee water-air, alcohol- water, ad alcohol-air, respectively. The NWs themselves do ot have sufficiet affiity to be 1

2 able to spread o the water surface because of the hydrophobic part of Si molecules. However, alcohol ca spread itself because alcohol reders the surface hydrophilic ad have the positive S. Therefore, the NWs diluted i alcohol ca spread o the water surface. 2

3 Fig. S2. Chemical structures of the ioic liquid ad UV crosslikable moomer exhibitig a mechaism of io-gel polymerizatio. To apply the io-gel gate dielectric, a photo patterable solutio cosistig of poly(ethylee glycol) diacrylate [PEGDA] crosslikig moomer, 2-hydroxy-2-methylpropiopheoe [HOMPP] photo iitiator, ad 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfoyl) imide [EMIM][TFSI] ioic liquid is drop-casted oto Au electrodes/si NWs/PI structure ad a square-pattered film mask was placed o the solutio. The, UV exposure o the solutio geerated free radicals from [HOMPP] that iitiated polymerizatio of acrylate ed groups o PEG derivatives. Furthermore, sice the pattered io-gel was covaletly boded to the PI substrate durig the UV crosslikig, the Si NWs were protected from exteral damages ad bedig stress. 3

4 Fig. S3. The OM images of (a) radomly aliged Si NWs with 40 μm i the average legth ad (b) directioally aliged Si NWs with 20 μm i the average legth. 4

5 Fig. S4. The SEM image of the multi-chael FET with 37 Si NWs exhibitig tilted agles of Si NWs from a perpedicular lie betwee electrodes. 5

6 Fig. S5. The trasfer curves of flexible Si NWs FETs as a fuctio of bedig cycle; the aliged umber of Si NWs were (a) 37 ad (b) 58, respectively. 6

7 Defiitio 1. Total correctio factor (k) Eve though the Si NWs were aliged ui-directioally, the θ (tilted agles of Si NWs from a perpedicular lie betwee electrodes) of Si NWs are ot idetical each other. Whe θ is ot 90, the chael legth (=the legth of Si NWs liked source ad drai electrodes) is ot same as the distace betwee source ad drai electrodes. So, the correctio factor of cosθ i, where θ i is the tilted agle of i th Si NW, eeds to be cosidered to calculate electrical properties of the FETs. If NWs are located betwee electrodes ad NWs have differet tilted agles to electrodes (θ 1 θ 2 θ ), the legths of NWs are differet ad equatio for calculatig the mobility of FET should be complicatedly modified. μ h = L/(W C d V ds ) g m k = L/((d ) C d V ds ) g m cosθ i i Where, d= mea diameter of Si NWs, = the umber of Si aowires located betwee electrodes, k= total correctio factor ( ) cosθ i i To simply solve this problem, it is assumed that the legths of NWs are equal (set a stadard chael legth to gap distace of electrodes, 10 μm, i this study) ad the losses i legths are compesated by the correctio factor. If a NW located betwee electrodes is tilted about θ 1 from stadard lie, the ratio of real legth of NW ad stadard legth is equal to 7

8 cosθ 1. Whe of NWs with differet tilted agles are haged, the total correctio factor (k) ca be idicated as cosθ i i. I this paper, the correctio factor was applied to calculate the electrical property of the Flexible Multi-Chael Si NW FETs. 8

9 Experimetal sectio Preparatio of Si NWs moolayer: The p-type Si NWs with 122 m i the average diameter ad 20 μm i the average legth were sythesized by a MCE process from a boro-doped p- type Si wafer (< 100 >, 1-10 Ω cm) followig our previous work 23 ad the stepwise reactio is described below: - Cathode reactio The oble metal acts as cathodic reactio site ad H 2 O 2 is reduced by oble metal. H 2 O 2 + 2Ag + 2H + 2H 2 O + 2Ag + [1] - Aode reactio Whe Si is i the tetravalet state, Si + 4Ag + + 4HF SiF 4 + 4Ag + 4H + SiF 4 + 2HF H 2 SiF 6 [2] [3] Whe Si is i the divalet state, Si + 4HF 2 SiF HF + H 2 + 2e [4] The mixed reactio of tetravalet ad divalet dissolutio is as follows: Si + 6HF + h + H 2 SiF 6 + H H 2 - The overall reactios are as follows: 2 H 2 O 2 + Ag + H + H 2 O + Ag + [5] [6] + Si + 6HF + Ag + H 2 SiF 6 + H + + Ag H 2 [7] = Si + 2 H 2 O 2 + 6HF H 2 O + H 2 SiF H 2 [8] 9

10 At the iterface betwee Ag ad etchat, Ag is oxidized by H 2 O 2 (cathode reactio) ad forms Ag + io i the viciity of the Ag film. This Ag + io is subsequetly reduced back to Ag (aode reactio) preferably oto the surface of Ag film istead of Si, sice Ag is more electroegative tha Si. Meawhile, Si is oxidized by HF resultig i the formatio of Si NWs. 1 Prepared Si NWs were dispersed i isopropyl alcohol (IPA) with 0.05% dilute hydrazie hydrate (N 2 H 4 ). To alig the Si NWs o a 15 μm-thick PI substrate, the LB aligmet method was applied. The Si NWs i IPA were dropped o DI water i a square dish ad the moolayer of Si NWs with the ui-directio was formed o the water surface. I this process, the umber of Si NWs was cotrolled by adjustig the desity of Si NWs i a dispersio solutio. A dispersio solutio with the desity of NWs/mL was used for the devices liked by 15, 22, ad 29 Si NWs, while the solutio with the desity of NWs/mL was used for the devices liked by 37, 46, ad 58 Si NWs. The PI, cleaed with acetoe, methaol, ad DI water for 10 mi each, was put ito the square dish ad Si NWs moolayer floated o Di water was dipped up ad trasferred o the PI substrate. Drop cast method for the sparsely distributed Si NWs aligmet : The Si NWs-dispersed solutio flowed o a 30 -tilted PI substrate i order for Si NWs to be aliged i parallel ad dried at room temperature to elimiate IPA solvet ad residual DI water. Depositio of Au electrodes ad positioig a io-gel: 100 m-thick Au layer was deposited o the aliged Si NWs as source ad drai electrodes usig a stecil mask with a 10 μm gap by e-beam evaporatio. To form a flexible GI o the Au electrodes ad Si NWs structure, 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulgoyl) imide [EMIM][TFSI] ioic liquid, poly(ethylee glycol) diacrylate [PEGDA] moomer, ad 2-hydroxy-2- methylpropiopheoe [HOMPP] UV cross-likig iitiator were mixed up at a ratio of 10:1:1 (w/w) ad the mixture was dropped o the Au/Si NWs/PI structure. After droppig the mixture, a pattered mask was located o it ad UV light (365 m, 100 mw/cm 2 ) shoe o the structure for 8 s, followed by risig with IPA to remove a ucured mixture. The morphologies of Si NWs ad the structure of the device were ivestigated by usig a scaig electro microscope (SEM, S-5000 HITACHI) ad a optical microscope. The crystallographic characteristics of the sythesized Si NWs were aalyzed by usig a scaig trasmissio electro microscope (STEM, JEM-ARM 200F). The I-V characteristics of the multi-chael Si NWs FETs were measured by usig a Agilet semicoductor parameter aalyzer (Agilet B1500A Agilet Techologies). The flexibility of the device was verified 10

11 by usig a bedig machie (Flexible Materials Tester, Hasug Systems Ic). Refereces 1. Z. P. Huag, N. Geyer, P. Werer, J. D. Boor ad U. Gösele, Adv. Mater., 2011, 23,