{"176051":{"#nid":"176051","#data":{"type":"news","title":"Self-Assembled Monolayers Create P-N Junctions in Graphene Films","body":[{"value":"\u003Cp\u003EThe electronic properties of graphene films are directly affected by the characteristics of the substrates on which they are grown or to which they are transferred. Researchers are taking advantage of this to create graphene p-n junctions by transferring films of the promising electronic material to substrates that have been patterned by compounds that are either strong electron donors or electron acceptors.\u003C\/p\u003E\u003Cp\u003EA low temperature, controllable and stable method has been developed to dope graphene films using self-assembled monolayers (SAM) that modify the interface of graphene and its support substrate. Using this concept, a team of researchers at the Georgia Institute of Technology has created graphene p-n junctions \u2013 which are essential to fabricating devices \u2013 without damaging the material\u2019s lattice structure or significantly reducing electron\/hole mobility.\u003C\/p\u003E\u003Cp\u003EThe graphene was grown on a copper film using chemical vapor deposition (CVD), a process that allows synthesis of large-scale films and their transfer to desired substrates for device applications. The graphene films were transferred to silicon dioxide substrates that were functionalized with the self-assembled monolayers.\u003C\/p\u003E\u003Cp\u003EInformation about creating graphene p-n junctions using self-assembled monolayers was presented on November 28, 2012 at the Fall Meeting of the Materials Research Society. Papers describing aspects of the work were also published in September 2012 in the journals \u003Cem\u003EACS Applied Materials \u0026amp; Interfaces\u003C\/em\u003E and the \u003Cem\u003EJournal of Physical Chemistry C\u003C\/em\u003E. Funding for the research came from the National Science Foundation, through the Georgia Tech Materials Research Science and Engineering Center (MRSEC) and through separate research grants.\u003C\/p\u003E\u003Cp\u003E\u201cWe have been successful at showing that you can make fairly well doped p-type and n-type graphene controllably by patterning the underlying monolayer instead of modifying the graphene directly,\u201d said \u003Ca href=\u0022http:\/\/www.chbe.gatech.edu\/faculty\/henderson\u0022\u003EClifford Henderson\u003C\/a\u003E, a professor in the Georgia Tech \u003Ca href=\u0022http:\/\/www.chbe.gatech.edu\/\u0022\u003ESchool of Chemical \u0026amp; Biomolecular Engineering\u003C\/a\u003E. \u201cPutting graphene on top of self-assembled monolayers uses the effect of electron donation or electron withdrawal from underneath the graphene to modify the material\u2019s electronic properties.\u201d\u003C\/p\u003E\u003Cp\u003EThe Georgia Tech research team working on the project includes faculty members, postdoctoral fellows and graduate students from three different schools. In addition to Henderson, professors who are part of the team include Laren Tolbert from the School of Chemistry and Biochemistry and Samuel Graham from the Woodruff School of Mechanical Engineering.\u0026nbsp; The project team also includes Hossein Sojoudi, a postdoctoral fellow, and Jose Baltazar, a graduate research assistant.\u003C\/p\u003E\u003Cp\u003ECreating n-type and p-type doping in graphene \u2013 which has no natural bandgap \u2013 has led to development of several approaches. Scientists have substituted nitrogen atoms for some of the carbon atoms in the graphene lattice, compounds have been applied to the surface of the graphene, and the edges of graphene nanoribbons have been modified. However, most of these techniques have disadvantages, including disruption of the lattice \u2013 which reduces electron mobility \u2013 and long-term stability issues.\u003C\/p\u003E\u003Cp\u003E\u201cAny time you put graphene into contact with a substrate of any kind, the material has an inherent tendency to change its electrical properties,\u201d Henderson said. \u201cWe wondered if we could do that in a controlled way and use it to our advantage to make the material predominately n-type or p-type. This could create a doping effect without introducing defects that would disrupt the material\u2019s attractive electron mobility.\u201d\u003C\/p\u003E\u003Cp\u003EUsing conventional lithography techniques, the researchers created patterns from different silane materials on a dielectric substrate, usually silicon oxide. The materials were chosen because they are either strong electron donors or electron acceptors. When a thin film of graphene is placed over the patterns, the underlying materials create charged sections in the graphene that correspond to the patterning.\u003C\/p\u003E\u003Cp\u003E\u201cWe were able to dope the graphene into both n-type and p-type materials through an electron donation or withdrawal effect from the monolayer,\u201d Henderson explained. \u201cThat doesn\u2019t lead to the substitutional defects that are seen with many of the other doping processes. The graphene structure itself is still pristine as it comes to us in the transfer process.\u201d\u003C\/p\u003E\u003Cp\u003EThe monolayers are bonded to the dielectric substrate and are thermally stable up to 200 degrees Celsius with the graphene film over them, Sojoudi noted. The Georgia Tech team has used 3-Aminopropyltriethoxysilane (APTES) and perfluorooctyltriethoxysilane (PFES) for patterning. In principle, however, there are many other commercially-available materials that could also create the patterns.\u003C\/p\u003E\u003Cp\u003E\u201cYou can build as many n-type and p-type regions as you want,\u201d Sojoudi said. \u201cYou can even step the doping controllably up and down. This technique gives you control over the doping level and what the dominant carrier is in each region.\u201d\u003C\/p\u003E\u003Cp\u003EThe researchers used their technique to fabricate graphene p-n junctions, which was verified by the creation of field-effect transistors (FET). Characteristic I-V curves indicated the presence of two separate Dirac points, which indicated an energy separation of neutrality points between the p and n regions in the graphene, Sojoudi said.\u003C\/p\u003E\u003Cp\u003EThe group uses chemical vapor deposition to create thin films of graphene on copper foil. A thick film of PMMA was spin-coated atop the graphene, and the underlying copper was then removed. The polymer serves as a carrier for the graphene until it can be placed onto the monolayer-coated substrate, after which it is removed.\u003C\/p\u003E\u003Cp\u003EBeyond developing the doping techniques, the team is also exploring new precursor materials that could allow CVD production of graphene at temperatures low enough to permit fabrication directly on other devices. That could eliminate the need for transferring the graphene from one substrate to another.\u003C\/p\u003E\u003Cp\u003EA low-cost, low-temperature means of producing graphene could also allow the films to find broader applications in displays, solar cells and organic light-emitting diodes, where large sheets of graphene would be needed.\u003C\/p\u003E\u003Cp\u003E\u201cThe real goal is to find ways to make graphene at lower temperatures and in ways that allow us to integrate it with other devices, either silicon CMOS or other materials that couldn\u2019t tolerate the high temperatures required for the initial growth,\u201d Henderson said. \u201cWe are looking at ways to make graphene into a useful electronic or opto-electronic material at low temperatures and in patterned forms.\u201d\u003C\/p\u003E\u003Cp\u003E\u003Cem\u003EThis material is based on work supported by the National Science Foundation (NSF) under Grants CHE-0822697, CHE-0848833 and CMMI-0927736 and the Georgia Tech Materials Research Science and Engineering Center (MRSEC). The content of this article is solely the responsibility of the authors and does not necessarily represent the official views of the NSF.\u003C\/em\u003E\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003ECITATIONS\u003C\/strong\u003E: Sojoudi, Hossein, Creating Graphene p-n Junctions Using Self-Assembled Monolayers, \u003Cem\u003EACS Applied Materials \u0026amp; Interfaces\u003C\/em\u003E, \u003Ca href=\u0022http:\/\/www.dx.doi.org\/10.1021\/am301138v\u0022\u003Edx.doi.org\/10.1021\/am301138v\u003C\/a\u003E and Baltazar, Jose, Facile Formation of Graphene P-N Junctions Using Self-Assembled Monolayers, \u003Cem\u003EThe Journal of Physical Chemistry C\u003C\/em\u003E, \u003Ca href=\u0022http:\/\/www.dx.doi.org\/10.1021\/jp3045737\u0022\u003Edx.doi.org\/10.1021\/jp3045737\u003C\/a\u003E.\u003Cbr \/\u003E\u003Cbr \/\u003E\u003Cstrong\u003EResearch News\u003C\/strong\u003E\u003Cbr \/\u003E\u003Cstrong\u003EGeorgia Institute of Technology\u003C\/strong\u003E\u003Cbr \/\u003E\u003Cstrong\u003E177 North Avenue\u003C\/strong\u003E\u003Cbr \/\u003E\u003Cstrong\u003EAtlanta, Georgia\u0026nbsp; 30332-0181\u003C\/strong\u003E\u003Cbr \/\u003E\u003Cbr \/\u003E\u003Cstrong\u003EMedia Relations Contact\u003C\/strong\u003E: John Toon (404-894-6986)(\u003Ca href=\u0022mailto:jtoon@gatech.edu\u0022\u003Ejtoon@gatech.edu\u003C\/a\u003E).\u003Cbr \/\u003E\u003Cstrong\u003EWriter\u003C\/strong\u003E: John Toon\u003C\/p\u003E","summary":null,"format":"limited_html"}],"field_subtitle":"","field_summary":[{"value":"\u003Cp\u003EResearchers are creating graphene p-n junctions by transferring films of the electronic material to substrates that have been patterned by compounds that are either strong electron donors or electron acceptors.\u003C\/p\u003E","format":"limited_html"}],"field_summary_sentence":[{"value":"Researchers have developed a new way to create graphene p-n junctions."}],"uid":"27303","created_gmt":"2012-12-09 17:14:19","changed_gmt":"2016-10-08 03:13:22","author":"John Toon","boilerplate_text":"","field_publication":"","field_article_url":"","dateline":{"date":"2012-12-10T00:00:00-05:00","iso_date":"2012-12-10T00:00:00-05:00","tz":"America\/New_York"},"extras":[],"hg_media":{"176011":{"id":"176011","type":"image","title":"Self Assembled Monolayers","body":null,"created":"1449179022","gmt_created":"2015-12-03 21:43:42","changed":"1475894819","gmt_changed":"2016-10-08 02:46:59","alt":"Self Assembled Monolayers","file":{"fid":"195861","name":"graphene-monolayer147.jpg","image_path":"\/sites\/default\/files\/images\/graphene-monolayer147_0.jpg","image_full_path":"http:\/\/hg.gatech.edu\/\/sites\/default\/files\/images\/graphene-monolayer147_0.jpg","mime":"image\/jpeg","size":1937468,"path_740":"http:\/\/hg.gatech.edu\/sites\/default\/files\/styles\/740xx_scale\/public\/images\/graphene-monolayer147_0.jpg?itok=-RNEo4cp"}},"176021":{"id":"176021","type":"image","title":"Self Assembled Monolayers2","body":null,"created":"1449179022","gmt_created":"2015-12-03 21:43:42","changed":"1475894819","gmt_changed":"2016-10-08 02:46:59","alt":"Self Assembled Monolayers2","file":{"fid":"195862","name":"graphene-monolayer212.jpg","image_path":"\/sites\/default\/files\/images\/graphene-monolayer212_0.jpg","image_full_path":"http:\/\/hg.gatech.edu\/\/sites\/default\/files\/images\/graphene-monolayer212_0.jpg","mime":"image\/jpeg","size":1682738,"path_740":"http:\/\/hg.gatech.edu\/sites\/default\/files\/styles\/740xx_scale\/public\/images\/graphene-monolayer212_0.jpg?itok=4pF_8e2h"}},"176031":{"id":"176031","type":"image","title":"Self Assembled Monolayers3","body":null,"created":"1449179022","gmt_created":"2015-12-03 21:43:42","changed":"1475894819","gmt_changed":"2016-10-08 02:46:59","alt":"Self Assembled Monolayers3","file":{"fid":"195863","name":"graphene-monolayer184.jpg","image_path":"\/sites\/default\/files\/images\/graphene-monolayer184_0.jpg","image_full_path":"http:\/\/hg.gatech.edu\/\/sites\/default\/files\/images\/graphene-monolayer184_0.jpg","mime":"image\/jpeg","size":1920980,"path_740":"http:\/\/hg.gatech.edu\/sites\/default\/files\/styles\/740xx_scale\/public\/images\/graphene-monolayer184_0.jpg?itok=pWkHJ69z"}},"176041":{"id":"176041","type":"image","title":"Self Assembled Monolayers4","body":null,"created":"1449179022","gmt_created":"2015-12-03 21:43:42","changed":"1475894819","gmt_changed":"2016-10-08 02:46:59","alt":"Self Assembled Monolayers4","file":{"fid":"195864","name":"graphene-monolayers25.jpg","image_path":"\/sites\/default\/files\/images\/graphene-monolayers25_0.jpg","image_full_path":"http:\/\/hg.gatech.edu\/\/sites\/default\/files\/images\/graphene-monolayers25_0.jpg","mime":"image\/jpeg","size":2457955,"path_740":"http:\/\/hg.gatech.edu\/sites\/default\/files\/styles\/740xx_scale\/public\/images\/graphene-monolayers25_0.jpg?itok=lxhF8wYW"}}},"media_ids":["176011","176021","176031","176041"],"groups":[{"id":"1188","name":"Research Horizons"}],"categories":[{"id":"141","name":"Chemistry and Chemical Engineering"},{"id":"149","name":"Nanotechnology and Nanoscience"}],"keywords":[{"id":"52431","name":"Clifford Henderson"},{"id":"429","name":"graphene"},{"id":"52411","name":"p-n junction"},{"id":"167750","name":"School of Chemical \u0026 Biomolecular Engineering"},{"id":"166928","name":"School of Chemistry and Biochemistry"},{"id":"169538","name":"self assembled monolayer"},{"id":"7528","name":"transistors"}],"core_research_areas":[{"id":"39451","name":"Electronics and Nanotechnology"},{"id":"39471","name":"Materials"}],"news_room_topics":[],"event_categories":[],"invited_audience":[],"affiliations":[],"classification":[],"areas_of_expertise":[],"news_and_recent_appearances":[],"phone":[],"contact":[{"value":"\u003Cp\u003EJohn Toon\u003C\/p\u003E\u003Cp\u003EResearch News\u003C\/p\u003E\u003Cp\u003E(404) 894-6986\u003C\/p\u003E\u003Cp\u003E\u003Ca href=\u0022mailto:jtoon@gatech.edu\u0022\u003Ejtoon@gatech.edu\u003C\/a\u003E\u003C\/p\u003E","format":"limited_html"}],"email":["jtoon@gatech.edu"],"slides":[],"orientation":[],"userdata":""}}}