對(duì)于普通材料，沿一個(gè)方向的晶格分別隨著其他正交方向的壓縮或拉伸而擴(kuò)展或收縮。泊松比（ν）是用于量化物理屬性的參數(shù)，在大多數(shù)情況下為正值。但對(duì)于某些特殊材料（所謂“膨脹材料”），ν可以為負(fù)。負(fù)泊松比（NPR）引起人們極大的興趣，因?yàn)榫哂蠳PR的材料類(lèi)型通常具有更強(qiáng)的韌性、抗剪切性和有效的降噪能力，從而可以實(shí)現(xiàn)許多新穎的應(yīng)用，如航空航天和國(guó)防等。從歷史上看，對(duì)NPR的研究大多是在體膨脹結(jié)構(gòu)上進(jìn)行的，進(jìn)一步擴(kuò)展到納米材料后，發(fā)現(xiàn)了更多有趣的現(xiàn)象。如，近來(lái)在碳納米管和金屬納米板中發(fā)現(xiàn)了NPR。另外，石墨烯等二維（2D）材料有望應(yīng)用于多個(gè)領(lǐng)域，而其也已通過(guò)特殊工程實(shí)現(xiàn)了NPR，可切成納米帶、引入空位缺陷、在極高溫度下產(chǎn)生周期性多孔和波紋彎曲等。但2D材料的NPR機(jī)制如何尚不清楚。

來(lái)自湖南大學(xué)的秦光照和鄭州大學(xué)的秦真真基于第一性原理方法，合作研究了石墨烯、硅烯、h-BN、h-GaN、h-SiC和h-BAs等新型二維蜂窩狀材料在單軸應(yīng)變下的機(jī)械響應(yīng)，預(yù)測(cè)上述材料在沿扶手椅方向時(shí)均存在負(fù)泊松比現(xiàn)象。盡管它們?cè)爻煞植煌銷(xiāo)PR均由鍵角的反常增加導(dǎo)致。這種鍵角的反常增加無(wú)法通過(guò)傳統(tǒng)層面的幾何結(jié)構(gòu)和力學(xué)響應(yīng)的觀點(diǎn)來(lái)解釋（如基于經(jīng)驗(yàn)勢(shì)函數(shù)的經(jīng)典分子動(dòng)力學(xué)模擬研究）。該工作通過(guò)對(duì)應(yīng)力調(diào)控下的關(guān)鍵幾何參數(shù)及軌道雜化作用的變化趨勢(shì)進(jìn)行分析，從電子結(jié)構(gòu)層面闡明了鍵角反常增加及NPR的底層物理機(jī)制。該機(jī)制同樣可適用于其他具有NPR現(xiàn)象的納米結(jié)構(gòu)。

Negative Poisson’s ratio in two-dimensional honeycomb structures

Guangzhao Qin and Zhenzhen Qin

Negative Poisson’s ratio （NPR） in auxetic materials is of great interest due to the typically enhanced mechanical properties， which enables plenty of novel applications. In this paper， by employing first-principles calculations， we report the emergence of NPR in a class of two-dimensional honeycomb structures （graphene， silicene， h-BN， h-GaN， h-SiC， and h-BAs）， which are distinct from all other known auxetic materials. They share the same mechanism for the emerged NPR despite the different chemical composition， which lies in the increased bond angle （θ）。 However， the increase of θ is quite intriguing and anomalous， which cannot be explained in the traditional point of view of the geometry structure and mechanical response， for example， in the framework of classical molecular dynamics simulations based on empirical potential. We attribute the counterintuitive increase of θ and the emerged NPR fundamentally to the strain-modulated electronic orbital coupling and hybridization. It is proposed that the NPR phenomenon can also emerge in other nanostructures or nanomaterials with similar honeycomb structure. The physical origin as revealed in our study deepens the understanding on the NPR and would shed light on future design of modern nanoscale electromechanical devices with special functions based on auxetic nanomaterials and nanostructures.

責(zé)任編輯:pj