{"title":"Current-voltage plots reveal details of the energy level alignment during photoelectron spectroscopy experiments","authors":"","doi":"10.1016/j.apsadv.2024.100643","DOIUrl":null,"url":null,"abstract":"<div><div>The photoelectron current <span><math><msub><mi>I</mi><mi>s</mi></msub></math></span> emitted from samples exposed to soft X-rays, such as is the case during X-ray photoelectron spectroscopy (XPS) analyses, is dominated by secondary electrons (SE) with energies not exceeding several eVs. Because of that, both the magnitude and the direction of <span><math><msub><mi>I</mi><mi>s</mi></msub></math></span> is highly sensitive to the applied sample bias <span><math><msub><mi>V</mi><mi>s</mi></msub></math></span> with the magnitude of just a few volts. By measuring current-voltage characteristics for a series of samples with different work functions <span><math><msub><mi>ϕ</mi><mrow><mi>S</mi><mi>A</mi></mrow></msub></math></span> a clear correlation between <span><math><msub><mi>ϕ</mi><mrow><mi>S</mi><mi>A</mi></mrow></msub></math></span> and the shape of the <span><math><mrow><msub><mi>I</mi><mi>s</mi></msub><mo>−</mo><msub><mi>V</mi><mi>s</mi></msub></mrow></math></span> curves is demonstrated. While all <span><math><mrow><msub><mi>I</mi><mi>s</mi></msub><mo>−</mo><msub><mi>V</mi><mi>s</mi></msub></mrow></math></span> plots have a characteristic “reversed S” shape, a clear and consistent shift towards positive <span><math><msub><mi>V</mi><mi>s</mi></msub></math></span> values is observed with increasing <span><math><msub><mi>ϕ</mi><mrow><mi>S</mi><mi>A</mi></mrow></msub></math></span>. The effect is explained by variations in the contact potential <span><math><msub><mi>V</mi><mi>c</mi></msub></math></span> established between the sample and the spectrometer. For all samples there is an excellent agreement between the <span><math><msub><mi>V</mi><mi>c</mi></msub></math></span> values (derived by ultraviolet photoelectron spectroscopy from the SE cut-offs) and the critical <span><math><msub><mi>V</mi><mi>s</mi></msub></math></span> value at which the photocurrent begins to drop. Hence, simple to perform current-voltage measurements, provide a unique insight into the details of the energy level alignment between sample and the spectrometer. In particular, the sign and the magnitude of the contact potential as well as relative changes in the sample work function can be determined. The knowledge of these parameters is often essential for correct interpretation of XPS spectra.</div></div>","PeriodicalId":34303,"journal":{"name":"Applied Surface Science Advances","volume":null,"pages":null},"PeriodicalIF":7.5000,"publicationDate":"2024-09-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.sciencedirect.com/science/article/pii/S2666523924000710/pdfft?md5=e7539ecdd61a3d201d4b16c346018b53&pid=1-s2.0-S2666523924000710-main.pdf","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Applied Surface Science Advances","FirstCategoryId":"1085","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S2666523924000710","RegionNum":0,"RegionCategory":null,"ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"CHEMISTRY, PHYSICAL","Score":null,"Total":0}
引用次数: 0
Abstract
The photoelectron current emitted from samples exposed to soft X-rays, such as is the case during X-ray photoelectron spectroscopy (XPS) analyses, is dominated by secondary electrons (SE) with energies not exceeding several eVs. Because of that, both the magnitude and the direction of is highly sensitive to the applied sample bias with the magnitude of just a few volts. By measuring current-voltage characteristics for a series of samples with different work functions a clear correlation between and the shape of the curves is demonstrated. While all plots have a characteristic “reversed S” shape, a clear and consistent shift towards positive values is observed with increasing . The effect is explained by variations in the contact potential established between the sample and the spectrometer. For all samples there is an excellent agreement between the values (derived by ultraviolet photoelectron spectroscopy from the SE cut-offs) and the critical value at which the photocurrent begins to drop. Hence, simple to perform current-voltage measurements, provide a unique insight into the details of the energy level alignment between sample and the spectrometer. In particular, the sign and the magnitude of the contact potential as well as relative changes in the sample work function can be determined. The knowledge of these parameters is often essential for correct interpretation of XPS spectra.
暴露在软 X 射线下的样品(例如在 X 射线光电子能谱(XPS)分析过程中)发出的光电子电流 Is 主要由能量不超过几 eV 的次级电子(SE)构成。因此,Is 的大小和方向对施加的样品偏压 Vs 非常敏感,其大小仅为几伏。通过测量一系列具有不同工作函数 ϕSA 的样品的电流-电压特性,可以清楚地看出 ϕSA 与 Is-Vs 曲线形状之间的相关性。虽然所有的 Is-Vs 曲线图都具有 "反向 S "形状的特征,但随着 ϕSA 的增大,Vs 值明显一致地向正值偏移。样品和光谱仪之间建立的接触电势 Vc 的变化可以解释这种效应。对于所有样品,Vc 值(通过紫外光电子能谱从 SE 截断点得出)与光电流开始下降的临界 Vs 值之间非常一致。因此,通过简单的电流-电压测量,可以深入了解样品与光谱仪之间能级排列的细节。尤其是可以确定接触电势的符号和大小,以及样品功函数的相对变化。了解这些参数往往对正确解读 XPS 光谱至关重要。