{"id":173,"date":"2018-10-25T17:43:17","date_gmt":"2018-10-25T17:43:17","guid":{"rendered":"https:\/\/commons.mtholyoke.edu\/aidalalab\/?page_id=173"},"modified":"2019-06-11T16:53:56","modified_gmt":"2019-06-11T16:53:56","slug":"time-resolved-transport-measurements","status":"publish","type":"page","link":"https:\/\/commons.mtholyoke.edu\/aidalalab\/time-resolved-transport-measurements\/","title":{"rendered":"Time-Resolved Transport Measurements"},"content":{"rendered":"<p>Kelvin Probe Force Microscopy records the electrical potential at the surface of the sample.\u00a0 One way to think about how it works is to consider the tip and the sample as a capacitor.\u00a0 If there is a potential difference between the sample and tip, there will be an electric field, and the tip will experience a force.\u00a0 We actually oscillate the tip by applying a small ac electrical signal at the resonance frequency, leading to an amplitude of oscillation.\u00a0 A DC feedback loop then alters the DC voltage of the tip until the applied voltage is equal to the potential at the surface of the sample, minimizing the force the tip feels and minimizing the amplitude of oscillation.\u00a0 By plotting the applied DC voltage at each point that minimizes the oscillation, we record the potential of the device.\u00a0 Most commonly, researchers use this technique to measure the workfunction of different materials with nanoscale resolution.\u00a0 Any contributions to the electric potential at the surface will contribute to the recorded voltage of the feedback loop, which includes trapped charges and intentionally applied voltages.\u00a0 Figure 1 shows a typical topography and potential image of a device that has two gold electrodes on either side and a channel of the organic semiconductor P3HT between them.\u00a0 The left electrode is grounded, and the right electrode is near 3 V.\u00a0 Use the slider to shift between the topography and potential image.<\/p>\n<div class=\"postbox h5p-sidebar\">\n<div class=\"h5p-action-bar-settings h5p-panel\"><code><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-3\" class=\"h5p-iframe\" data-content-id=\"3\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Height and Potential\"><\/iframe><\/div><\/code><\/div>\n<\/div>\n<div class=\"postbox h5p-sidebar\"><\/div>\n<p>We have developed a technique that measures the real-time injection and extraction of charges on the scale of milliseconds, a relevant time-scale for many disordered non-traditional semiconductors with transport that can be dominated by traps.\u00a0 By using an FET geometry with a backgate (see Figure 2), and grounding the source and drain electrodes, we can record the real-time screening of the potential applied to the gate.\u00a0 We step the gate voltage, which results in an immediate change in the potential measured by the tip.\u00a0 Over time, charges rush in or out of the film, screening this potential, as illustrated in Figure 2.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-770 \" src=\"https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-content\/uploads\/sites\/483\/2019\/04\/ChargeMotionAnimationv01-e1560270647886.gif\" alt=\"\" width=\"554\" height=\"296\" \/><\/p>\n<p><em>Figure 2: Cartoon of the time resolved KPFM setup. An FET with a backgate allows us to position the tip above the materials that we are studying. By stepping the gate voltage (negative in this case), charges (positive) rush into the film from the grounded electrodes in response to the electric field, eventually screening the field from the tip.<\/em>&nbsp;<\/p>\n<figure id=\"attachment_833\" aria-describedby=\"caption-attachment-833\" style=\"width: 360px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-833\" src=\"https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-content\/uploads\/sites\/483\/2019\/06\/controltrKPFM-300x189.jpg\" alt=\"\" width=\"360\" height=\"227\" srcset=\"https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-content\/uploads\/sites\/483\/2019\/06\/controltrKPFM-300x189.jpg 300w, https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-content\/uploads\/sites\/483\/2019\/06\/controltrKPFM.jpg 716w\" sizes=\"auto, (max-width: 360px) 100vw, 360px\" \/><figcaption id=\"caption-attachment-833\" class=\"wp-caption-text\"><em>Figure 3: tr-KPFM signal from a control sample with no semiconducting film.<\/em><\/figcaption><\/figure>\n<p>Figure 3 shows a control sample, without a semiconducting film.\u00a0 The dotted black line is the voltage applied to the gate, -7V in this case.\u00a0 The yellow line is the potential the tip records above the grounded gold electrode. Unsurprisingly, the line is constant at zero, because the electrons in the gold move so quickly in response to the gate voltage that we do not see any change in potential.\u00a0 When the tip is instead placed over the silicon dioxide insulator, the blue line results.\u00a0 The tip records a negative step in potential, following gate voltage.\u00a0 We fit an exponential to the rise time and find a time constant of 3ms, giving us our time resolution for this experiment.<\/p>\n<figure id=\"attachment_836\" aria-describedby=\"caption-attachment-836\" style=\"width: 492px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-836\" src=\"https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-content\/uploads\/sites\/483\/2019\/06\/trKPFMdata-300x136.png\" alt=\"\" width=\"492\" height=\"223\" srcset=\"https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-content\/uploads\/sites\/483\/2019\/06\/trKPFMdata-300x136.png 300w, https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-content\/uploads\/sites\/483\/2019\/06\/trKPFMdata-768x348.png 768w, https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-content\/uploads\/sites\/483\/2019\/06\/trKPFMdata-1024x464.png 1024w, https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-content\/uploads\/sites\/483\/2019\/06\/trKPFMdata.png 1438w\" sizes=\"auto, (max-width: 492px) 100vw, 492px\" \/><figcaption id=\"caption-attachment-836\" class=\"wp-caption-text\"><em>Fgure 4: Typical tr-KPFM result from a hole majority carrier (P3HT)<\/em><\/figcaption><\/figure>\n<p>Figure 4 shows data from a P3HT film.\u00a0 As the gate voltage is stepped negative, the tip records a momentary negative potential, which returns to zero as the holes are injected into the film from the gold electrodes, screening the potential.\u00a0 As the gate voltage is stepped back to zero, there is an initial surplus of holes in the film, resulting in a momentary positive potential, which returns to zero as holes exit the film.\u00a0 For the positive gate voltage, there is an initial positive potential recorded by the tip that returns to zero as more holes leave the film.\u00a0 Returning the gate voltage to zero results in a momentary negative potential recorded by the tip as there are initially too few holes in the film.\u00a0 In response to the gate voltage change, holes are injected and the potential returns to zero.\u00a0 We can see by eye that it takes longer to extract holes than inject holes. This can be understood by considering the role that traps play in transport in this sample.\u00a0 As carriers enter the film, some fall into traps.\u00a0 This can happen quickly, so injecting the holes happens quickly.\u00a0 When the holes must be extracted, some are caught in these traps, and so it takes longer for the holes to exit the film.\u00a0 For more information, see Moscatello et al., <em>Organic Electronics<\/em>,\u00a0<strong>41<\/strong>, 26 (2017), which compares the results on an electron majority carrier and examines the effect of bias stress.<\/p>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Kelvin Probe Force Microscopy records the electrical potential at the surface of the sample.\u00a0 One way to think about how it works is to consider the tip and the sample as a capacitor.\u00a0 If there is a potential difference between the sample and tip, there will be an electric field, and the tip will experience&hellip;<\/p>\n","protected":false},"author":1030,"featured_media":673,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_monsterinsights_skip_tracking":false,"footnotes":""},"class_list":["post-173","page","type-page","status-publish","has-post-thumbnail","hentry"],"_links":{"self":[{"href":"https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-json\/wp\/v2\/pages\/173","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-json\/wp\/v2\/users\/1030"}],"replies":[{"embeddable":true,"href":"https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-json\/wp\/v2\/comments?post=173"}],"version-history":[{"count":14,"href":"https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-json\/wp\/v2\/pages\/173\/revisions"}],"predecessor-version":[{"id":838,"href":"https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-json\/wp\/v2\/pages\/173\/revisions\/838"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-json\/wp\/v2\/media\/673"}],"wp:attachment":[{"href":"https:\/\/commons.mtholyoke.edu\/aidalalab\/wp-json\/wp\/v2\/media?parent=173"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}